A compact high-repetition-rate passively mode-locked resonator structure and fiber laser

Abstract

The application discloses a compact high-repetition-frequency passively mode-locked resonant cavity structure and a fiber laser. The resonant cavity comprises a first ferrule, a semiconductor saturable absorber mirror, a first sleeve, a graded-index lens, a second sleeve, a gain fiber, a second ferrule and a dielectric film. The semiconductor saturable absorber mirror is arranged on the end face of the first ferrule, the first ferrule is connected to one end of the graded-index lens through the first sleeve, the other end of the graded-index lens is connected to the second ferrule through the second sleeve, the dielectric film is arranged on the other end face of the second ferrule, and the gain fiber is located in the second ferrule. By arranging the graded-index lens between the gain fiber and the semiconductor saturable absorber mirror, direct contact between the semiconductor saturable absorber mirror and the end face of the fiber is avoided, good heat dissipation of the resonant cavity can be realized while high-repetition-frequency mode-locked pulse output is obtained, and damage of the fiber laser caused by long-time mode-locked operation is avoided.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, specifically relating to a compact high repetition rate passive mode-locked resonant cavity structure and a fiber laser. Background Technology

[0002] High-repetition-rate laser pulses, characterized by short pulse intervals and large longitudinal mode spacing, hold significant research value and application potential in numerous fields, including optical frequency measurement, high-speed optical sampling, ultra-high-speed optical communication, precision machining, and biomedicine. Meanwhile, thanks to the small size, light weight, and flexibility of optical fibers, fiber lasers, using fiber as the gain medium, offer advantages such as compact structure and strong heat dissipation. Furthermore, their low manufacturing cost and high optical-to-optical conversion efficiency make them highly favored in scientific research and commercial applications, making fiber lasers the preferred research subject for obtaining highly reliable, high-pulse-quality laser sources.

[0003] Mode-locking is a crucial technique for generating picosecond or even femtosecond ultrashort pulses. Depending on the method, it can be categorized into active mode-locking and passive mode-locking. Active mode-locking involves introducing an active element into the laser resonator to periodically modulate the amplitude or phase of the intracavity optical field, creating a fixed phase relationship between different modes within the resonator. Passive mode-locking, on the other hand, involves adding a natural or equivalent saturable absorber to the resonator. Through the saturable absorption effect, strong and weak light signals experience the same gain and significantly different losses when simultaneously traveling back and forth within the resonator. This results in stable oscillations of the strong light pulse while the weak light signal gradually disappears, creating a stable self-starting laser pulse within the cavity, thus forming a pulse with a period of... The ultrashort laser pulse sequence (L is the resonant cavity length, c is the speed of light). Compared with the costly and complex active mode-locked laser, the passive mode-locked laser can obtain mode-locked pulse output without external modulation. Therefore, its structure is simpler and more compact, its performance is more stable, and it is easier to achieve high repetition rate femtosecond mode-locked pulse output.

[0004] Semiconductor saturable absorber mirrors are widely used in passive mode-locking technology. They are formed by growing special semiconductor materials directly on a semiconductor Bragg mirror and growing a mirror on the top layer or using the semiconductor-air interface as a mirror. The two mirrors are combined to form a Fabry-Perot cavity, resulting in a mirror with saturable absorption performance. The modulation depth and bandwidth of saturable absorption can be adjusted by changing the thickness of the absorber and adjusting the reflectivity of the two mirrors. Semiconductor saturable absorber mirrors are widely used in passive mode-locked fiber lasers due to their compact structure and self-starting mode-locking characteristics (Song D, Yin K, Miao R, et al. Theoretical and experimental investigations of dispersion-managed, polarization-maintaining 1-GHz mode-locked fiber lasers[J]. Optics Express, 2023, 31(2):1916-1930.). Furthermore, due to their low unsaturated loss and modulation depth, they are beneficial for suppressing Q-switching instability in high repetition frequency fundamental frequency passive mode-locking. To achieve high-repetition-rate passively mode-locked pulse output in the GHz range, the resonant cavity length of fiber lasers is limited to the centimeter range. Furthermore, due to the gain coefficient limitations of the gain fiber, most current high-repetition-rate passively mode-locked fiber laser designs primarily achieve efficient optical reflection by tightly fitting the end face of the gain fiber to the end face of the semiconductor saturable absorber mirror. However, this method easily damages the surface of the semiconductor saturable absorber mirror, thereby disrupting the stable mode-locked state (e.g., Q-switching instability). When Q-switching instability occurs, the high peak power generated by the fiber laser will further damage the semiconductor saturable absorber mirror. Additionally, this design hinders heat dissipation from the semiconductor saturable absorber mirror; the high heat accumulated during prolonged mode-locked operation will permanently damage the mode-locking element, affecting the performance of the high-repetition-rate passively mode-locked fiber laser. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, the purpose of this invention is to provide a compact high repetition rate passive mode-locked resonant cavity structure. The resonant cavity structure is a Fabry-Perot cavity structure. By adding a graded refractive index lens between the semiconductor saturable absorber mirror and the gain fiber, the semiconductor saturable absorber mirror does not directly contact the fiber end face. This achieves good heat dissipation of the resonant cavity while obtaining high repetition rate mode-locked pulse output, thus avoiding damage to the fiber laser caused by long-term mode-locked operation.

[0006] The present invention is achieved by at least one of the following technical solutions.

[0007] A compact high repetition rate passive mode-locked resonant cavity structure, the resonant cavity comprising a first ferrule, a semiconductor saturable absorber mirror, a first sleeve, a graded refractive index lens, a second sleeve, a gain fiber, a second ferrule, and a dielectric film;

[0008] The semiconductor saturable absorber mirror is disposed on the end face of the first ferrule. The first ferrule is connected to one end of the graded refractive index lens through a first sleeve. The other end of the graded refractive index lens is connected to the second ferrule through a second sleeve. A dielectric film is disposed on the other end face of the second ferrule. The gain fiber is located inside the second ferrule.

[0009] Furthermore, the first sleeve is disposed outside the first ferrule and the graded refractive index lens, and the second sleeve is disposed outside the second ferrule and the graded refractive index lens.

[0010] Furthermore, the distances between the gradient refractive index lens, the semiconductor saturable absorber lens, and the gain fiber are all adjustable.

[0011] Furthermore, the refractive index of the material of the gradient refractive index lens gradually decreases radially, causing continuous refraction when light propagates axially, thus achieving smooth and continuous convergence of light to a single point.

[0012] Furthermore, the dielectric film has a reflectivity of greater than 70% for signal light and a transmittance of greater than 70% for pump light.

[0013] Furthermore, the modulation depth of the semiconductor saturable absorber mirror is 1% to 30%.

[0014] Furthermore, the length of the gain fiber is 1 cm to 9 cm.

[0015] Furthermore, the gain fiber is a rare-earth ion-doped fiber, and the doped rare-earth ions include one or more of erbium, ytterbium, thulium, and holmium.

[0016] Furthermore, the gain fiber is fixed inside a size-matched second ferrule using epoxy resin and polished at the end face.

[0017] A fiber laser containing the aforementioned compact high repetition rate passive mode-locked resonant cavity structure includes: a wavelength division multiplexer, a pump source, an optical isolator, and a resonant cavity;

[0018] The wavelength division multiplexer is used to couple the pump light generated by the pump source into the resonant cavity and output the generated signal light to the outside of the resonant cavity. The optical isolator is connected to the wavelength division multiplexer.

[0019] Compared with existing technologies, the beneficial effects of the present invention are as follows:

[0020] This invention provides a compact high repetition rate passive mode-locked resonant cavity structure. By placing a graded refractive index lens between the semiconductor saturable absorber mirror and the gain fiber, direct contact between the semiconductor saturable absorber mirror and the fiber end face is avoided. This suppresses the damage caused by the heat generated by the gain fiber on the semiconductor saturable absorber mirror during long-term mode-locked operation of the laser. It can achieve good heat dissipation of the resonant cavity while obtaining high repetition rate mode-locked pulse output. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be considered as limiting the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of a high repetition rate passive mode-locked resonant cavity structure according to an embodiment of the present invention;

[0023] Figure 2 This is a schematic diagram of the propagation path of light in a graded refractive index lens according to an embodiment of the present invention;

[0024] Figure 3 This is a schematic diagram of the high repetition rate passively mode-locked fiber laser structure according to an embodiment of the present invention.

[0025] in:

[0026] 1-First ferrule; 2-Semiconductor saturable absorber mirror; 3-First sleeve; 4-Graded refractive index lens; 5-Second sleeve; 6-Gain fiber; 7-Second ferrule; 8-Dielectric film; 9-Resonant cavity; 10-Wavelength division multiplexer; 11-Pump source; 12-Isolator. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0028] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the technical solutions of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that the specific embodiments described herein are only for explaining this application and are not intended to limit this application.

[0029] Example 1

[0030] like Figure 1 As shown, this embodiment provides a compact high repetition frequency passive mode-locked resonant cavity structure. The resonant cavity 9 includes a first ferrule 1, a semiconductor saturable absorber mirror 2, a first sleeve 3, a graded refractive index lens 4, a second sleeve 5, a gain fiber 6, a second ferrule 7, and a dielectric film 8.

[0031] The semiconductor saturable absorber mirror 2 is disposed on the end face of the first ferrule 1. The first ferrule 1 is connected to one end of the graded refractive index lens 4 through the first sleeve 3. The other end of the graded refractive index lens 4 is connected to the second ferrule 7 through the second sleeve 5. The dielectric film 8 is disposed on the other end face of the second ferrule 7. The gain fiber 6 is located inside the second ferrule 7. The first sleeve 3 is disposed outside the first ferrule 1 and the graded refractive index lens 4. The second sleeve 5 is disposed outside the second ferrule 7 and the graded refractive index lens 4.

[0032] In practical applications, the resonant cavity 9 is a Fabry-Perot cavity structure with an overall length of <10 cm, which can achieve high repetition frequency mode-locked pulse output of >1 GHz.

[0033] The distances between the graded refractive index lens 4 and the semiconductor saturable absorber mirror 2 on the first ferrule 1 and the gain fiber 6 in the second ferrule 7 are adjustable to control the size of the light spot incident on the semiconductor saturable absorber mirror 2 and adjust the optimal coupling efficiency. Figure 2 The diagram shows the propagation path of light in the gradient refractive index lens 4 according to an embodiment of the present invention. The refractive index of the material of the gradient refractive index lens 4 gradually decreases in the radial direction, which enables continuous refraction when light is transmitted in the axial direction, so as to achieve smooth and continuous convergence of light to a point and realize the self-converging function.

[0034] The dielectric film 8 is a two-color dielectric film deposited on one end face of the second ferrule 7 by plasma sputtering. Its film thickness is 13 micrometers, the reflection center wavelength is 1064 nanometers, the reflection bandwidth is 1010-1080 nanometers, the reflectivity is greater than 85%, the transmission center wavelength is 976 nanometers, and the transmittance is greater than 90%.

[0035] The semiconductor saturable absorber mirror 2 is fixed to one end face of the first ferrule 1. It is formed by directly growing a special semiconductor material onto a semiconductor Bragg mirror, and then growing another mirror on top. The combination of these two mirrors creates a Fabry-Perot cavity, thus constituting a mirror with saturable absorption performance. Its center wavelength is 1040 nm, its reflection bandwidth is 1020-1100 nm, its area is 1×1 mm, its thickness is 450 μm, its unsaturated absorption is 8%, its modulation depth is 5%, its unsaturated loss is 3%, and its saturated flux is 40 μJ / cm². 2 The relaxation time is 1 picosecond, and the damage threshold is 3 mJ / cm². 2 .

[0036] The gain fiber 6 is a ytterbium rare earth ion-doped fiber with a length of 5 cm. The core diameter and cladding diameter are 4 micrometers and 125 micrometers, respectively. After it is fixed inside the second ferrule 7 with epoxy resin, the two ends of the second ferrule 7 need to be vertically polished.

[0037] Both the first ferrule 1 and the second ferrule 7 are ceramic ferrules with an inner diameter of 125 micrometers, which matches the cladding diameter of the gain fiber 9. The outer diameter is equal to the outer diameter of the graded refractive index lens 3, which is 2.5 millimeters. The length of the first ferrule 1 is 1 centimeter, and the length of the second ferrule 7 is equal to the length of the ytterbium-doped rare earth ion gain fiber 6, which is 5 centimeters.

[0038] Both the first and second sleeves are ceramic sleeves, with a length of 2 cm and an inner diameter of 2.5 mm. Their inner diameters match the outer diameters of the first insert 1, the second insert 7, and the gradient refractive index lens 4.

[0039] like Figure 3 As shown, the present invention also provides a high repetition rate passively mode-locked fiber laser, including a wavelength division multiplexer 10, a pump source 11, an optical isolator 12, and the resonant cavity 9 described in the above embodiment. The pump source 11 is a 976nm single-mode semiconductor laser; the wavelength division multiplexer 10 is used to couple the pump light generated by the pump source 11 into the ultrashort resonant cavity 9 and output the generated signal light outside the resonant cavity 9; the optical isolator 12 is connected to the wavelength division multiplexer 10 and is used to prevent the return light from affecting the output of the high repetition rate mode-locked pulse.

[0040] Example 2

[0041] like Figure 1 As shown, the compact high repetition rate passive mode-locked resonant cavity structure provided in this embodiment is the same as that in Embodiment 1, both being typical Fabry-Perot cavity structures. However, the material parameters of the intracavity gain fiber, dispersive film, semiconductor saturable absorber mirror, etc., are different, resulting in different center spectra, spectral bandwidth, and repetition frequencies of the high repetition rate mode-locked pulse output.

[0042] The dielectric film 8 is a two-color dielectric film deposited on one end face of the second insert 7 by plasma sputtering. Its film thickness is 16 micrometers, the reflection center wavelength is 1550 nanometers, the reflection bandwidth is 1480-1700 nanometers, the reflectivity is greater than 90%, the transmission center wavelength is 976 nanometers, and the transmittance is greater than 90%.

[0043] The semiconductor saturable absorber mirror 2 has a center wavelength of 1550 nm, a reflection bandwidth of 1450-1580 nm, an area of ​​1×1 mm, a thickness of 450 μm, an unsaturated absorption of 7%, a modulation depth of 3%, an unsaturated loss of 4%, and a saturated flux of 15 μJ / cm². 2 The relaxation time is 10 picoseconds, and the damage threshold is 800 μJ / cm². 2 .

[0044] The gain fiber uses erbium-ytterbium co-doped phosphate fiber, with a length of 6 cm, a core diameter of 6 μm, and a cladding diameter of 125 μm. After being fixed inside the second ferrule 7 with epoxy resin, both ends of the second ferrule 7 need to be vertically polished. The length of the second ferrule is equal to the length of the gain fiber, which is 6 cm.

[0045] like Figure 3 As shown, when the resonant cavity in this embodiment is used in a high repetition rate passively mode-locked fiber laser, the pump source used is a 976nm single-mode semiconductor laser.

[0046] Example 3

[0047] like Figure 1 As shown, the compact high repetition rate passive mode-locked resonant cavity structure provided in this embodiment is the same as that in Embodiment 1, both being typical Fabry-Perot cavity structures. However, the material parameters of the intracavity gain fiber, dispersive film, semiconductor saturable absorber mirror, etc., are different, resulting in different center spectra, spectral bandwidth, and repetition frequencies of the high repetition rate mode-locked pulse output.

[0048] The dielectric film 8 is a two-color dielectric film deposited on one end face of the second ferrule by plasma sputtering. Its film thickness is 18 micrometers, the reflection center wavelength is 1950 nanometers, the reflection bandwidth is 1850-2050 nanometers, the reflectivity is greater than 90%, the transmission center wavelength is 1570 nanometers, and the transmittance is greater than 95%.

[0049] The semiconductor saturable absorber mirror 2 has a center wavelength of 2000 nm, a reflection bandwidth of 1890-2060 nm, an area of ​​1×1 mm, a thickness of 450 μm, an unsaturated absorption of 20%, a modulation depth of 12%, an unsaturated loss of 8%, and a saturated flux of 65 μJ / cm². 2The relaxation time is 10 picoseconds, and the damage threshold is 2 mJ / cm². 2 .

[0050] The gain fiber uses thulium-doped silica fiber, is 4 cm long, and has a core diameter of 5 μm and a cladding diameter of 125 μm. After being fixed inside the second ferrule 7 with epoxy resin, both ends of the second ferrule 7 need to be vertically polished. The length of the second ferrule is equal to the length of the gain fiber, which is 4 cm.

[0051] like Figure 3 As shown, when the resonant cavity in this embodiment is used in a high repetition rate passively mode-locked fiber laser, the pump source used is a 1570nm single-mode semiconductor laser.

[0052] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0053] The above embodiments of the present invention are merely examples for clearly illustrating the present invention and are not intended to limit the implementation of the present invention. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively describe all possible implementations here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A compact high repetition rate passively mode-locked resonator structure, characterized in that, The resonant cavity (9) comprises a first ferrule (1), a semiconductor saturable absorber mirror (2), a first sleeve (3), a graded-index lens (4), a second sleeve (5), a gain fiber (6), a second ferrule (7), and a dielectric film (8). The semiconductor saturable absorber mirror (2) is arranged on the end face of the first ferrule (1), the first ferrule (1) is connected to one end of the graded-index lens (4) through the first sleeve (3), the other end of the graded-index lens (4) is connected to the second ferrule (7) through the second sleeve (5), and the dielectric film (8) is arranged on the other end face of the second ferrule (7), and the gain fiber (6) is located in the second ferrule (7). The material refractive index of the graded-index lens (4) gradually decreases along the radial direction, so that continuous refraction occurs when the light is transmitted along the axial direction, the light is smoothly and continuously converged to a point, and the distance between the graded-index lens (4) and the semiconductor saturable absorber mirror (2) and the gain fiber (6) can be adjusted. The first sleeve (3) is arranged outside the first ferrule (1) and the graded-index lens (4), and the second sleeve (5) is arranged outside the second ferrule (7) and the graded-index lens (4). The modulation depth of the semiconductor saturable absorber mirror (2) is 1% to 30%. The length of the gain fiber (6) is 1 cm to 9 cm.

2. A compact high repetition rate passively mode-locked resonator structure according to claim 1, characterized in that, The reflectivity of the dielectric film (8) to signal light is greater than 70%, and the transmittance of the dielectric film (8) to pump light is greater than 70%.

3. A compact high repetition rate passively mode-locked resonator structure according to claim 1, characterized in that, The gain fiber (6) is a rare earth ion doped fiber, and the doped rare earth ions include one or more of erbium, ytterbium, thulium and holmium.

4. A compact high repetition rate passively mode-locked resonator structure according to claim 1, characterized in that, The gain fiber (6) is fixed in the size-matched second ferrule (7) using epoxy resin, and is polished at the end face.

5. A fiber laser comprising a compact high repetition rate passively mode-locked resonator structure according to claim 1, characterized in that, It comprises: a wavelength division multiplexer (10), a pump source (11), an optical isolator (12), and the resonant cavity (9). The wavelength division multiplexer (10) is used for coupling the pump light generated by the pump source (11) into the resonant cavity (9) and outputting the generated signal light out of the resonant cavity (9), and the optical isolator (12) is connected to the wavelength division multiplexer (10).

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