Fiber laser for realizing mode locking by using Fabry-Perot-like cavity and configuration method

Abstract

The invention belongs to the technical field of optical fiber lasers, and discloses an optical fiber laser for realizing mode locking by using a Fabry-Perot-like cavity and a configuration method of the optical fiber laser, and the method comprises the following steps: analyzing and calculating the dependence of the light intensity transmittance of the Fabry-Perot-like cavity on an incident polarization state based on a Fresnel equation theory; the quasi-saturable absorption effect in the nonlinear environment of the optical fiber is analyzed; and the mode-locked fiber laser configuration is realized by using the saturated absorption effect generated in the Fabry-Perot-like cavity. According to the invention, the mode-locked fiber laser configuration is realized by using the Fabry-Perot-like cavity, the dependence of the light intensity transmittance of the Fabry-Perot-like cavity on polarization is analyzed theoretically, the saturable absorption effect-like capability in a fiber laser system environment is proved in experiments, and mode locking is realized. The method does not need a special material, and is not limited by a material power threshold value. Compared with a polarization rotation mode locking method, the polarization rotation mode locking method does not need to use a polarization dependent isolator, so that the probability of optical loss and device damage is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber lasers, and in particular relates to an optical fiber laser that realizes mode locking by using a Fabry-Perot-like cavity and a configuration method thereof. Background Art

[0002] Fiber lasers are a type of laser that uses rare earth-doped glass optical fiber as a gain medium. They are highly regarded in the field of laser technology due to their good beam quality, directionality, stability, and monochromaticity. In addition, fiber lasers also have the advantages of high efficiency, low threshold, flexible wavelength selectivity, good thermal stability, low loss, no need for water cooling, and compact layout. Fiber lasers are widely used in various scientific and industrial fields due to their excellent performance. In the field of material processing, thanks to its high precision and high power, it can be used for cutting, welding, and marking of various materials such as metals, ceramics, and polymers. In addition, fiber lasers have become an indispensable tool in scientific research, covering a variety of fields such as spectroscopy, particle manipulation, and quantum optics. Due to its wide impact, the performance and stability improvement of fiber lasers has far-reaching significance in modern science and industry.

[0003] According to the time characteristics of the fiber laser output beam, it can be divided into two categories, namely continuous wave fiber laser and pulse fiber laser. Mode locking is a common technology for generating pulsed lasers, which can produce ultrashort pulses.

[0004] In fiber lasers, mode locking is usually achieved by using the saturable absorption effect, and there are two main generation mechanisms. The first is to use the nonlinear effects of the fiber, such as self-phase modulation to cause polarization rotation, and to cooperate with polarization-dependent isolators to induce the saturated absorption effect. The second involves the interaction between the material itself and the light field, which belongs to the category of saturable absorbers. Substances used for optical saturable absorbers include two-dimensional materials such as graphene, and three-dimensional micro-nano materials such as ternary transition metal carbides.

[0005] Nonlinear polarization rotation mode-locked fiber lasers can generate ultrashort pulses. Due to the high pulse energy density, the impact on the polarization-dependent isolator is large, making it an easily damaged device. Finding alternative devices is of great significance.

[0006] Through the above analysis, the problems and defects of the prior art are as follows:

[0007] (1) Since the pulse energy density generated by the nonlinear polarization rotation mode-locked fiber laser is relatively high, the impact on the polarization-dependent isolator is relatively large, making it a device that is easily damaged.

[0008] (2) The traditional technology is to add materials to the jumper flat head, which is prone to cause damage threshold problems due to the existence of the material, and also has the problem of low transmittance due to the absorption of light by the material. Summary of the invention

[0009] In order to overcome the problems existing in the related art, the embodiments disclosed in the present invention provide a fiber laser and a configuration method for realizing mode locking using a Fabry-Perot-like cavity. The technical solution is as follows:

[0010] The present invention is implemented by using a Fabry-Perot-like cavity to realize mode-locked fiber laser configuration method, comprising:

[0011] S1. Based on the theoretical analysis of the Fresnel equation, the dependence of the light intensity transmittance of the Fabry-Perot cavity on the incident polarization state is calculated;

[0012] S2. Analyze the quasi-saturable absorption effect in the nonlinear environment of optical fiber;

[0013] S3. Utilize the saturation absorption effect produced in a Fabry-Perot-like cavity to achieve a mode-locked fiber laser configuration.

[0014] The method also includes: calculating the total light intensity transmittance after light in polarization states corresponding to different angles and different thicknesses is incident on the Fabry-Perot-like cavity.

[0015] The method also includes: adjusting the polarization controller so that the tip of the pulse has the highest transmittance through the polarizer, and the pulse width is continuously narrowed under the effect of the transmission difference, eventually forming a stable locked mode.

[0016] Furthermore, the polarization controller realizes a quasi-saturable absorption effect.

[0017] The method also includes: using a Fabry-Perot-like cavity and a polarization-independent isolator to replace the polarization-dependent isolator to achieve mode locking.

[0018] The method further includes: defining a P polarized photoelectric vector parallel to an incident plane, wherein the incident plane is a plane determined by an incident direction and a reflection direction; and defining an S polarized photoelectric vector perpendicular to the incident plane. According to the Fresnel formula, the transmittance and reflectance of the P polarized light and the S polarized light are determined by the following equations:

[0019]

[0020] In the formula, r p is the reflectivity of P polarized light, r s is the reflectivity of S polarized light, t p For r p Light intensity transmittance, t s For r s is the light intensity transmittance, n1 and n2 are the refractive indices of different media, i1 and i2 are the incident angle and refraction angle respectively.

[0021] Light is reflected multiple times at the interface of two media, resulting in multi-beam interference. Based on multi-beam interference, the total intensity transmittance is calculated as follows:

[0022]

[0023] Where T is the total light intensity transmittance, δ is the phase difference, n is the refractive index of the cavity medium, r is the reflectivity of the internal cavity wall, h is the cavity thickness, and λ is the wavelength of the incident light in vacuum.

[0024] Another object of the present invention is to provide a fiber laser that uses a Fabry-Perot cavity to achieve mode locking. The fiber laser implements the fiber laser configuration method that uses a Fabry-Perot cavity to achieve mode locking. The fiber laser includes: a laser diode, a wavelength division multiplexing coupler, an erbium-doped fiber, an optical coupler, an angled jumper connector, and a polarization-independent isolator.

[0025] Furthermore, the pump laser enters the erbium-doped optical fiber through a wavelength division multiplexing coupler. After the light passes through the wavelength division multiplexing coupler, part of it is used as output, and the remaining light continues to pass through the Fabry-Perot-like cavity formed by the angled jumper connector; after the light passes through the angled jumper connector, it passes through a non-polarization-dependent optical isolator to ensure unidirectional propagation in the cavity; the light passes through a polarization controller to adjust the polarization state of the light in the cavity.

[0026] Furthermore, the polarization controller is connected to another port of the wavelength division multiplexing coupler to form a ring cavity.

[0027] In combination with all the above technical solutions, the beneficial effects of the present invention are as follows:

[0028] The present invention uses a Fabry-Perot-like cavity to achieve a mode-locked fiber laser configuration, theoretically analyzes the dependence of the light intensity transmittance of the Fabry-Perot-like cavity on polarization, and experimentally proves the ability to have a quasi-saturable absorption effect in a fiber laser system environment, thereby achieving mode locking. The advantage of this method is that it does not require special materials and is therefore not limited by the material power threshold. Compared with polarization rotation mode locking, the present invention does not require the use of a polarization-dependent isolator, thereby reducing the probability of optical loss and device damage. In addition, the present invention can further reduce the output pulse width by optimizing parameters such as cavity length and gain fiber.

[0029] The present invention proposes a fiber laser configuration that uses the saturation absorption effect generated in a Fabry-Perot-like cavity to achieve mode locking, which eliminates the need for specific materials and polarization-dependent isolators.

[0030] The present invention utilizes an angled jumper connector and can lock the mold without adding any material, thereby overcoming the problem of damage threshold caused by the existence of the material and avoiding the problem of low transmittance caused by the existence of the material absorbing light.

[0031] Based on the Fresnel equation theory, the present invention analyzes and calculates the dependence of the light intensity transmittance of a Fabry-Perot cavity-like cavity on the incident polarization state, analyzes the saturable absorption effect in the nonlinear environment of optical fiber, and experimentally verifies the replaceable polarization isolator with the ability of mode locking using common commercial optical fiber devices.

[0032] The present invention overcomes the technical difficulty that the polarization-dependent isolator of the conventional polarization rotation mode locking is easily damaged, and at the same time, compared with the conventional material mode locking, it also solves the problem of the material damage resistance threshold. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings herein are incorporated in and constitute a part of the specification, illustrate embodiments consistent with the present disclosure, and together with the description, serve to explain the principles of the present disclosure;

[0034] Figure 1 It is a flow chart of a method for configuring a fiber laser for achieving mode locking using a Fabry-Perot-like cavity provided by an embodiment of the present invention;

[0035] Figure 2 is a schematic diagram of refraction and reflection of light provided by an embodiment of the present invention;

[0036] Figure 3 Schematic diagram of bevel physical contact (bevel fiber jumper head) provided by an embodiment of the present invention;

[0037] Figure 4A is a schematic diagram of changes in the transmittance of P light and S light with the incident angle provided by an embodiment of the present invention;

[0038] Figure 4B is a schematic diagram of changes in the transmittance of P light and S light with thickness provided by an embodiment of the present invention;

[0039] Figure 5 is a schematic diagram of light intensity transmittance of different polarization states after passing through a Fabry-Perot-like cavity provided by an embodiment of the present invention;

[0040] Figure 6 is a schematic diagram of the quasi-saturable absorption effect of nonlinear polarization rotation mode locking provided by an embodiment of the present invention;

[0041] Figure 7 is a schematic diagram of the experimental optical path configuration provided by an embodiment of the present invention;

[0042] Fig. 8A is a schematic diagram of a pulse sequence provided by an embodiment of the present invention;

[0043] Figure 8B It is a schematic diagram of a single pulse provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0044] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific implementation disclosed below.

[0045] Embodiment 1: The method for configuring a fiber laser for achieving mode locking using a Fabry-Perot-like cavity provided in the embodiment of the present invention comprises the following steps:

[0046] S1. Based on the theoretical analysis of the Fresnel equation, the dependence of the light intensity transmittance of the Fabry-Perot cavity on the incident polarization state is calculated;

[0047] S2. Analyze the quasi-saturable absorption effect in the nonlinear environment of optical fiber;

[0048] S3. Utilize the saturation absorption effect produced in a Fabry-Perot-like cavity to achieve a mode-locked fiber laser configuration.

[0049] The fiber laser configuration method provided by the embodiment of the present invention further includes: calculating the total light intensity transmittance after the light of polarization states corresponding to different angles is incident on the Fabry-Perot-like cavity;

[0050] The polarization controller is adjusted so that the tip of the pulse has the highest transmittance through the polarizer. The pulse width is continuously narrowed due to the transmission difference, eventually forming a stable mode locking.

[0051] The polarization controller and polarizer provided by the embodiment of the present invention realize a quasi-saturable absorption effect.

[0052] The polarizer in the optical fiber provided in the embodiment of the present invention is realized by a polarization-dependent isolator.

[0053] The fiber laser configuration method provided by the embodiment of the present invention further includes: using a Fabry-Perot-like cavity and a polarization-independent isolator to replace the polarization-dependent isolator to achieve mode locking.

[0054] Embodiment 2: The fiber laser for achieving mode locking using a Fabry-Perot-like cavity provided in the embodiment of the present invention comprises: a laser diode, a wavelength division multiplexing coupler, an erbium-doped fiber, an optical coupler, an angled jumper connector, and a polarization-independent isolator.

[0055] The pump laser provided in the embodiment of the present invention enters the erbium-doped optical fiber through a wavelength division multiplexing coupler, and erbium ions serve as an active gain medium in the optical fiber; after the light passes through the wavelength division multiplexing coupler, part of the light is used as output, and the remaining light continues to pass through the Fabry-Perot-like cavity formed by the angled jumper connector; after the light passes through the angled jumper connector, it passes through a non-polarization-dependent optical isolator to ensure unidirectional propagation in the cavity; the light passes through a polarization controller to adjust the polarization state of the light in the cavity; finally, it is connected to another port of the wavelength division multiplexing coupler to form a ring cavity, and the total length of the ring cavity is 9m.

[0056] In order to further demonstrate the positive effects of the above embodiment, the present invention conducts the following experiments based on the above technical solution.

[0057] 1. Fabry-Perot-like cavity

[0058] like Figure 2 As shown in , when light propagates in different media, transmission and reflection occur at the interface. Define the P polarization photoelectric vector to be parallel to the incident plane, where the incident plane is the plane determined by the incident direction and the reflection direction, as shown in Figure 2 In the plane where the S-polarized light vector is perpendicular to the incident plane. According to the Fresnel formula, the transmittance and reflectance of P-polarized light and S-polarized light are determined by the following equations:

[0059]

[0060] In the formula, r p is the reflectivity of P polarized light, r s is the reflectivity of S polarized light, t p For r p Light intensity transmittance, t s For r s is the light intensity transmittance, n1 and n2 are the refractive indices of different media, i1 and i2 are the incident angle and refraction angle respectively.

[0061] Two parallel and facing high reflectivity surfaces can form a Fabry-Perot cavity, such as Figure 3 As shown in the figure, two patch cord heads (APC) with inclined end faces close to each other can form a cavity, which is called a Fabry-Perot cavity due to the low reflectivity of the end faces. In this configuration, light is reflected multiple times at the interface of the two media, resulting in multi-beam interference. Based on multi-beam interference, the total intensity transmittance can be calculated as follows:

[0062]

[0063] Where T is the total light intensity transmittance, δ is the phase difference, n is the refractive index of the cavity medium, r is the reflectivity of the internal cavity wall, h is the cavity thickness, and λ is the wavelength of the incident light in vacuum.

[0064] Substituting the reflectivity of P polarized light and S polarized light, the corresponding light intensity transmittance t can be obtained. p and t s ,The calculation results show that in general, their values ​​are not the same.

[0065] Figure 4A The light intensity transmittance changes with the incident angle at a certain thickness. It can be observed that when the incident angle is 30°, the transmittance difference between P-polarized light and S-polarized light is large. When the incident angle remains constant, the transmittance fluctuates periodically with the change of the cavity thickness, that is, the end face distance, such as Figure 4B As shown. The present invention knows that the polarization of light is determined by the size of two orthogonal components and the phase difference between them. When light passes through a Fabry-Perot cavity, the phase difference between P light and S light will not change, but the difference in transmission intensity causes the ratio of the two components to change, which in turn causes the polarization state to change. More importantly, the total light intensity transmittance will be different when light of different polarization states passes through it, or in other words, when light passes through a Fabry-Perot cavity, the total light intensity transmittance is related to the polarization state of the incident light. The present invention can prove this through calculation. For this purpose, the polarization state is first quantified. When the horizontal linear polarized light is 0° with the optical axis of the quarter wave plate, it is still horizontal linear polarized light. When the horizontal linear polarized light is 45° with the optical axis of the quarter wave plate, it is circularly polarized light. When the angle between the horizontal linear polarized light and the optical axis of the quarter wave plate changes from 0° to 45°, the corresponding transmitted light gradually evolves from linear polarized light to circularly polarized light through elliptically polarized light. If the angle continues to increase from 45° to 90°, the corresponding circularly polarized light evolves into vertical linear polarized light through elliptically polarized light. Therefore, in this process, each angle corresponds to a different polarization state. If the angle increases from 90° to 180°, the corresponding vertical linear polarized light evolves into horizontal polarized light, so the polarization state represented by 60° is the same as that of 120°.

[0066] The present invention calculates the total light intensity transmittance of light of polarization states corresponding to different angles after being incident on the Fabry-Perot cavity. In order to show the obvious difference, select Figure 4A The transmittances of P light and S light with an incident angle of 30° are 0.98 and 0.72 respectively. The calculation results show that the transmittance of light with different incident polarization states is different, such as Figure 5 As shown in the figure, it can be seen that the vertical polarized light has the highest light intensity transmittance after passing through the Fabry-Perot-like cavity. The reason is that the vertical polarization direction is the same as the P light direction, and for the cavity, the P light direction has the highest transmittance.

[0067] How to apply it to mode-locked lasers? The present invention first examines polarization rotation mode-locked lasers. When light pulses run in optical fibers, there is a nonlinear effect, which causes different polarizations of different parts of the pulses, such as Figure 6As shown. Adjust the polarization controller so that the tip of the pulse has the highest transmittance through the rear polarizer, and the edge transmittance is naturally lower. The pulse circulates continuously inside the cavity, and the pulse width is continuously narrowed under the effect of the transmission difference, eventually forming a stable mode locking. Here, the polarization controller and polarizer actually play a role of quasi-saturable absorption. The polarizer in the optical fiber is realized by a polarization-dependent isolator, such as Figure 6 As shown, the polarizer needs to absorb light energy in the horizontal direction to play a role. Sometimes due to factors such as disturbance, the polarization of the pulse changes when it reaches the polarizer, which may cause it to absorb huge energy, so the polarization-dependent isolator is prone to damage. If the present invention uses a Fabry-Perot cavity and a polarization-independent isolator to replace the polarization-dependent isolator, the same effect can be achieved, and because the loss of the Fabry-Perot cavity comes from reflection, it is not easy to be damaged.

[0068] 2. Experimental system and measurement results:

[0069] In the experimental optical path, 980nm LD is a laser diode; WDM is a wavelength division multiplexing coupler; EDF represents erbium-doped fiber; OC is an optical coupler; APC is an angled patch cord connector; and PI-ISO is a polarization-independent isolator.

[0070] The present invention uses common optical fiber devices to simply verify the theory. In the experiment, a commercially available inclined physical contact (APC) connector was selected, such as Figure 7 As shown, to achieve a quasi-saturation absorption function. The connector is tilted at an angle of 26°. A 980nm pump laser enters a 1.2m long erbium-doped fiber (EDF) through a wavelength division multiplexer (WDM), in which erbium ions serve as an active gain medium. After the light passes through the coupler, part of it is used as output, while the remaining light continues to pass through the Fabry-Perot-like cavity formed by the APC connector. After passing through the APC connector, the light passes through a non-polarization-dependent optical isolator to ensure unidirectional propagation in the cavity. The light then passes through a polarization controller to adjust the polarization state of the light in the cavity. Finally, it is connected to another port of the WDM to form a ring cavity. The total cavity length is 9m.

[0071] The experimental results show that the mode-locked pulse train was successfully generated, such as Fig. 8A As shown in Figure 1, the repetition frequency is 22.9MHz. The single pulse time domain width directly measured by the oscilloscope is 3.5ns, as shown in Figure 1. Figure 8B The oscilloscope model used in the experiment (ROHDE&SCHWARZ, RTO2012, 1GHz, 10GSa / s) was not used to measure the pulse width due to experimental conditions. The actual pulse width in the time domain may be in the order of femtoseconds.

[0072] The present invention theoretically analyzes the dependence of the light intensity transmittance of the Fabry-Perot cavity on polarization, and experimentally proves the ability to have a quasi-saturable absorption effect in the fiber laser system environment, and realizes mode locking. The advantage of this method is that it does not require special materials and is therefore not limited by the material power threshold. Compared with polarization rotation mode locking, the present invention does not require the use of polarization-dependent isolators, thereby reducing the probability of optical loss and device damage. In addition, by optimizing parameters such as cavity length and gain fiber, the output pulse width can be further reduced.

[0073] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principles of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for configuring a fiber laser using a Fabry-Perot-like cavity to achieve mode locking, characterized in that: The method includes: S1. Based on the theoretical analysis of the Fresnel equation, the dependence of the light intensity transmittance of the Fabry-Perot cavity on the incident polarization state is calculated; S2. Analyze the quasi-saturable absorption effect in the nonlinear environment of optical fiber; S3. Utilize the saturation absorption effect produced in a Fabry-Perot-like cavity to achieve a mode-locked fiber laser configuration.

2. The method for configuring a fiber laser using a Fabry-Perot-like cavity to achieve mode locking according to claim 1, characterized in that: The method also includes: calculating the total light intensity transmittance after light in polarization states corresponding to different angles and different thicknesses is incident on the Fabry-Perot-like cavity.

3. The method for configuring a fiber laser using a Fabry-Perot-like cavity to achieve mode locking according to claim 2, characterized in that: The method also includes: adjusting the polarization controller so that the tip of the pulse has the highest transmittance through the polarizer, and the pulse width is continuously narrowed under the effect of the transmission difference, eventually forming a stable locked mode.

4. The method for configuring a fiber laser using a Fabry-Perot-like cavity to achieve mode locking according to claim 3, characterized in that: The polarization controller realizes a quasi-saturable absorption effect.

5. The method for configuring a fiber laser using a Fabry-Perot-like cavity to achieve mode locking according to claim 4, characterized in that: The method also includes: using a Fabry-Perot-like cavity and a polarization-independent isolator to replace the polarization-dependent isolator to achieve mode locking.

6. The method for configuring a fiber laser using a Fabry-Perot-like cavity to achieve mode locking according to claim 5, characterized in that: The method further includes: defining a P polarized photoelectric vector parallel to an incident plane, wherein the incident plane is a plane determined by an incident direction and a reflection direction; and defining an S polarized photoelectric vector perpendicular to the incident plane. According to the Fresnel formula, the transmittance and reflectance of the P polarized light and the S polarized light are determined by the following equations: In the formula, r p is the reflectivity of P polarized light, r s is the reflectivity of S polarized light, t p For r p Light intensity transmittance, t s For r s is the light intensity transmittance, n1 and n2 are the refractive indices of different media, i1 and i2 are the incident angle and refraction angle respectively.

7. The method for configuring a fiber laser using a Fabry-Perot-like cavity to achieve mode locking according to claim 6, characterized in that: Light is reflected multiple times at the interface of two media, resulting in multi-beam interference. Based on multi-beam interference, the total intensity transmittance is calculated as follows: Where T is the total light intensity transmittance, δ is the phase difference, n is the refractive index of the cavity medium, r is the reflectivity of the internal cavity wall, h is the cavity thickness, and λ is the wavelength of the incident light in vacuum.

8. A fiber laser that uses a Fabry-Perot-like cavity to achieve mode locking, characterized in that: The fiber laser implements the fiber laser configuration method for realizing mode locking using a Fabry-Perot-like cavity as described in any one of claims 1 to 7, and the fiber laser comprises: a laser diode, a wavelength division multiplexing coupler, an erbium-doped fiber, an optical coupler, an angled jumper connector, and a polarization-independent isolator.

9. The fiber laser using a Fabry-Perot-like cavity to achieve mode locking according to claim 8, characterized in that: The pump laser enters the erbium-doped optical fiber through a wavelength division multiplexing coupler. After the light passes through the wavelength division multiplexing coupler, part of it is used as output, and the remaining light continues to pass through the Fabry-Perot cavity formed by the angled jumper connector; after passing through the angled jumper connector, the light passes through a non-polarization-dependent optical isolator to ensure unidirectional propagation in the cavity; the light passes through a polarization controller to adjust the polarization state of the light in the cavity.

10. The fiber laser using a Fabry-Perot-like cavity to achieve mode locking according to claim 9, characterized in that: The polarization controller is connected to another port of the wavelength division multiplexing coupler to form a ring cavity.

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