8-shaped cavity mode-locking neodymium-doped femtosecond fiber laser

Abstract

The invention relates to a fiber laser, and discloses an 8-shaped cavity mode-locking neodymium-doped femtosecond fiber laser, which comprises a 3 * 3 fiber coupler, a pumping source, a wavelength division multiplexer, a chirped fiber Bragg grating, a neodymium-doped fiber and a fiber isolator, a first end of the wavelength division multiplexer is connected with the pumping source, pump light generated by the pumping source is coupled into a resonant cavity, and a second end of the wavelength division multiplexer is connected with the fiber isolator. The second end of the wavelength division multiplexer is connected with the first end of the optical fiber coupler, and the third end of the wavelength division multiplexer is connected with the chirp fiber Bragg grating; the first end of the optical fiber coupler is sequentially connected with the wavelength division multiplexer and the chirp fiber Bragg grating to form a linear end, the second end and the third end of the optical fiber coupler are connected with the optical fiber isolator to form a main ring, and light is transmitted in the main ring in a one-way mode; the fourth end and the fifth end of the optical fiber coupler are connected with the neodymium-doped optical fiber to form a nonlinear amplification annular mirror; and the sixth end of the optical fiber coupler is an optical fiber output end. The device is compact in structure, stable and reliable.

Description

Technical Field

[0001] The invention belongs to the technical field of lasers, and particularly relates to an 8-cavity mode-locked neodymium-doped femtosecond fiber laser. Background Art

[0002] In recent years, two-photon microscopy has developed rapidly, and a stable and portable femtosecond laser operating at 0.9μm is urgently needed as a light source to improve imaging quality. In addition, pure blue light of about 460nm is obtained through frequency doubling technology. 0.9μm is the exclusive band of neodymium ions, so it is of great significance to develop a stable and compact mode-locked neodymium-doped femtosecond fiber laser.

[0003] Conventional figure-eight cavity structures are usually based on 2×2 fiber couplers, which may suffer from high mode-locking thresholds. A typical approach to overcome this problem is to introduce a non-reciprocal phase shifter inside the resonant cavity. However, this scheme utilizes a Faraday rotator and a wave plate, so that the phase shift element is not a fully fused fiber component, which may be sensitive to ambient temperature. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a compact, stable and reliable figure-8 cavity mode-locked neodymium-doped femtosecond fiber laser.

[0005] The 8-cavity mode-locked neodymium-doped femtosecond fiber laser provided by the present invention comprises a 3×3 fiber coupler, a pump source, a wavelength division multiplexer, a chirped fiber Bragg grating, a neodymium-doped fiber and a fiber isolator.

[0006] The pump source is used to generate pump light;

[0007] The chirped fiber Bragg grating is used to reflect the laser light, reflect the laser light back into the resonant cavity, and adjust the dispersion in the resonant cavity so that the total dispersion of the entire resonant cavity is approximately zero;

[0008] The first end of the wavelength division multiplexer is connected to the pump source to couple the pump light generated by the pump source into the resonant cavity, the second end of the wavelength division multiplexer is connected to the first end of the 3×3 fiber coupler, and the third end of the wavelength division multiplexer is connected to the chirped fiber Bragg grating;

[0009] The first end of the 3×3 fiber coupler is connected to the wavelength division multiplexer and the chirped fiber Bragg grating in sequence to form a linear end, and the linear end is used to input pump light and reflect laser light; the second end and the third end of the 3×3 fiber coupler are connected to the fiber isolator to form a main ring, and light is transmitted unidirectionally in the main ring; the fourth end and the fifth end of the 3×3 fiber coupler are connected to the neodymium-doped fiber to form a nonlinear amplifying ring mirror; the sixth end of the 3×3 fiber coupler is the fiber output end;

[0010] The neodymium-doped optical fiber is excited by the pump light to generate laser signal light.

[0011] Furthermore, the 3×3 optical fiber coupler is a 3×3 polarization-maintaining optical fiber coupler.

[0012] Furthermore, the splitting ratio of the 3×3 optical fiber coupler is uniform.

[0013] Furthermore, the fourth end, the fifth end and the sixth end of the 3×3 optical fiber coupler have a phase difference of 120° with each other.

[0014] Furthermore, the neodymium-doped optical fiber is a polarization-maintaining neodymium-doped optical fiber.

[0015] Furthermore, the neodymium-doped optical fiber is arranged close to the 3×3 optical fiber coupler.

[0016] Furthermore, the wavelength division multiplexer is a polarization-maintaining wavelength division multiplexer.

[0017] Furthermore, the 3×3 fiber coupler divides the light into two paths. The clockwise transmitted light is first amplified by the neodymium-doped fiber and then accumulates nonlinear phase shift in the nonlinear amplifying ring mirror; the counterclockwise transmitted light first accumulates nonlinear phase shift and finally amplifies by the neodymium-doped fiber. The two paths of light accumulate different nonlinear phase shifts related to the pulse intensity and interfere at the 3×3 fiber coupler. The center of the pulse with higher energy is transmitted and the two edges of the pulse with lower energy are reflected, thereby achieving mode locking.

[0018] Furthermore, two beams of light converge and interfere at the 3×3 fiber coupler and then enter the linear end and the main ring; the fiber isolator in the main ring allows the input light to be transmitted unidirectionally in the main ring back to the 3×3 fiber coupler; the chirped fiber Bragg grating at the linear end reflects the light back to the 3×3 fiber coupler, while compensating for the dispersion to further narrow the pulse; the laser reflected and unidirectionally transmitted back to the 3×3 fiber coupler enters the nonlinear amplifying ring mirror again, and thus reciprocates, the laser transmits oscillations in the resonant cavity, and is finally output through the fiber output end of the 3×3 fiber coupler.

[0019] Furthermore, the dispersion parameter of the chirped fiber Bragg grating can be selected to compensate for the total dispersion of the entire resonant cavity.

[0020] Compared with the prior art, the present invention has the following beneficial effects: through the design of the present invention, the laser structure is compact, stable and reliable, and the cost is low and the operation is efficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0022] Figure 1 A schematic structural diagram of a figure-8 cavity mode-locked neodymium-doped femtosecond fiber laser provided in an embodiment of the present invention.

[0023] The meanings of the symbols in the accompanying drawings are:

[0024] 1- pump source, 2- wavelength division multiplexer, 3- chirped fiber Bragg grating, 4- 3×3 fiber coupler, 5- neodymium-doped fiber, 6- fiber isolator. DETAILED DESCRIPTION

[0025] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] See also Figure 1 The figure-8 cavity mode-locked neodymium-doped femtosecond fiber laser provided in an embodiment of the present invention includes a 3×3 fiber coupler 4, a pump source 1, a wavelength division multiplexer 2, a chirped fiber Bragg grating 3, a neodymium-doped fiber 5 and a fiber isolator 6.

[0027] The pump source 1 provides a pump light source for generating pump light to achieve population inversion of neodymium ions in the neodymium-doped optical fiber 5, which is a necessary condition for laser output.

[0028] The chirped fiber Bragg grating 3 is used as a cavity mirror of the resonant cavity to reflect the laser and reflect the laser back into the resonant cavity; the dispersion parameter of the chirped fiber Bragg grating 3 can be selected to compensate for the total dispersion of the entire resonant cavity, and the dispersion in the resonant cavity is adjusted to make the total dispersion of the entire resonant cavity approximately zero, so that the pulse is not broadened due to dispersion, and femtosecond pulse laser is output. The chirped fiber Bragg grating 3 can also define the starting wavelength of the oscillator, effectively suppress the four-level radiation transition in the neodymium-doped fiber 5, and realize stable three-level system operation.

[0029] The first end of the wavelength division multiplexer 2 is connected to the pump source 1 to couple the pump light generated by the pump source into the resonant cavity; the second end of the wavelength division multiplexer 2 is connected to the first end of the 3×3 fiber coupler 4 to transmit the pump light and the signal light; the third end of the wavelength division multiplexer 2 is connected to the chirped fiber Bragg grating 3 to transmit the signal light.

[0030] The 3×3 fiber coupler 4 constitutes the entire figure-eight cavity structure. The first end of the 3×3 fiber coupler 4 is connected to the wavelength division multiplexer 2 and the chirped fiber Bragg grating 3 in sequence to form a linear end, which is used to input pump light and reflect laser light; the second and third ends of the 3×3 fiber coupler 4 are connected to the fiber isolator 6 to form a main ring, and light is transmitted unidirectionally in the main ring; the fourth and fifth ends of the 3×3 fiber coupler 4 are connected to the neodymium-doped fiber 5 to form a nonlinear amplifying ring mirror, which is a key structure for achieving mode locking; the sixth end of the 3×3 fiber coupler 4 is the fiber output end. In this embodiment, the splitting ratio of the 3×3 fiber coupler is uniform.

[0031] Preferably, the fourth end, the fifth end and the sixth end of the 3×3 optical fiber coupler 4 have a phase difference of 120° with each other, which provides a linear phase shift for the nonlinear amplifying loop mirror mode locking.

[0032] The neodymium-doped fiber 5 is an optical fiber doped with rare earth ions neodymium ions, and light amplification is achieved by using the gain mechanism caused by neodymium ions in the optical fiber. The condition for achieving light amplification is the inversion of the population of neodymium ions in the active optical fiber. When the outside world supplies energy to the neodymium-doped optical fiber (called excitation or pumping process) to make the neodymium ions in the optical fiber in a non-thermal equilibrium state, the population inversion can be achieved, so the pumping process is a necessary condition for light amplification. The neodymium-doped optical fiber 5 is excited by the pump light to generate laser signal light. The neodymium-doped optical fiber 5 is arranged close to the 3×3 optical fiber coupler 4 so that the nonlinear phase shifts accumulated in the nonlinear amplification ring mirror loop of the two paths of light are different, which is more conducive to mode locking.

[0033] The optical fiber isolator 6 enables the light to be transmitted unidirectionally in the main ring.

[0034] The laser of this embodiment adopts a full polarization-maintaining structure, the 3×3 fiber coupler is a 3×3 polarization-maintaining fiber coupler, the neodymium-doped fiber is a polarization-maintaining neodymium-doped fiber, and the wavelength division multiplexer is a polarization-maintaining wavelength division multiplexer.

[0035] The working principle of the laser provided in this embodiment is as follows: the pump light is output from the pump source 1, coupled into the resonant cavity by the wavelength division multiplexer 2, and enters the nonlinear amplifying ring mirror through the common port (second end) of the wavelength division multiplexer 2 through the fiber coupler 4, providing energy for the neodymium-doped fiber 5, and achieving the particle number inversion condition of the laser output. This extracavity pumping method is different from the direct pumping method of directly fusing the pump source connected to the wavelength division multiplexer with the neodymium-doped fiber, and is applicable to lasers with relatively close wavelengths of the pump light and the signal light, and is more effective. The light is divided into two paths through the fiber coupler 4. The clockwise transmitted light is first amplified by the neodymium-doped fiber 5, and then accumulates nonlinear phase shift in the nonlinear amplifying ring mirror; the counterclockwise transmitted light first accumulates nonlinear phase shift, and finally amplifies by the neodymium-doped fiber 5; the two paths of light accumulate different nonlinear phase shifts related to the pulse intensity, interfere at the fiber coupler 4, the pulse center with higher energy is transmitted and the two edges of the pulse with lower energy are reflected, thereby achieving mode locking. In addition, the fiber coupler 4 provides an additional linear phase shift of 120°, which is more conducive to the self-starting of the nonlinear amplifying loop mirror mode locking.

[0036] After the two beams of light converge and interfere at the fiber coupler 4, they enter the linear end and the main ring; the fiber isolator in the main ring allows the input light to be transmitted unidirectionally back to the fiber coupler 4 in the main ring; the chirped fiber Bragg grating 3 at the linear end acts as a cavity reflector to reflect the light back to the fiber coupler 4, while compensating for the dispersion to further narrow the pulse; the laser reflected and unidirectionally transmitted back to the fiber coupler 4 enters the nonlinear amplifying ring mirror again, and thus reciprocates, the laser transmits and oscillates in the resonant cavity, and is finally output through the fiber output end of the fiber coupler 4.

[0037] The figure-8 cavity mode-locked neodymium-doped femtosecond fiber laser designed in this embodiment is more concise, stable and low-cost.

[0038] The above is a detailed introduction to the figure-8 cavity mode-locked neodymium-doped femtosecond fiber laser disclosed in the embodiment of the present invention. Specific examples are used in this article to illustrate the principle and implementation mode of the present invention. The description of the above embodiments is only used to help understand the core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation mode and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. 8-cavity mode-locked neodymium-doped femtosecond fiber laser, characterized in that: It includes 3×3 fiber coupler, pump source, wavelength division multiplexer, chirped fiber Bragg grating, neodymium-doped fiber and fiber isolator. The pump source is used to generate pump light; The chirped fiber Bragg grating is used to reflect the laser light, reflect the laser light back into the resonant cavity, and adjust the dispersion in the resonant cavity so that the total dispersion of the entire resonant cavity is approximately zero; The first end of the wavelength division multiplexer is connected to the pump source to couple the pump light generated by the pump source into the resonant cavity, the second end of the wavelength division multiplexer is connected to the first end of the 3×3 fiber coupler, and the third end of the wavelength division multiplexer is connected to the chirped fiber Bragg grating; The first end of the 3×3 fiber coupler is connected to the wavelength division multiplexer and the chirped fiber Bragg grating in sequence to form a linear end, and the linear end is used to input pump light and reflect laser light; the second end and the third end of the 3×3 fiber coupler are connected to the fiber isolator to form a main ring, and light is transmitted unidirectionally in the main ring; the fourth end and the fifth end of the 3×3 fiber coupler are connected to the neodymium-doped fiber to form a nonlinear amplifying ring mirror; the sixth end of the 3×3 fiber coupler is the fiber output end; The neodymium-doped optical fiber is excited by the pump light to generate laser signal light.

2. The figure-8 cavity mode-locked neodymium-doped femtosecond fiber laser according to claim 1, characterized in that: The 3×3 optical fiber coupler is a 3×3 polarization-maintaining optical fiber coupler.

3. The figure-8 cavity mode-locked neodymium-doped femtosecond fiber laser according to claim 1, characterized in that: The 3×3 optical fiber coupler has a uniform light splitting ratio.

4. The figure-8 cavity mode-locked neodymium-doped femtosecond fiber laser according to claim 1, characterized in that: The fourth end, the fifth end and the sixth end of the 3×3 optical fiber coupler have a phase difference of 120° with each other.

5. The figure-8 cavity mode-locked neodymium-doped femtosecond fiber laser according to claim 1, characterized in that: The neodymium-doped optical fiber is a polarization-maintaining neodymium-doped optical fiber.

6. The figure-8 cavity mode-locked neodymium-doped femtosecond fiber laser according to claim 1, characterized in that: The neodymium-doped optical fiber is arranged close to the 3×3 optical fiber coupler.

7. The figure-8 cavity mode-locked neodymium-doped femtosecond fiber laser according to claim 1, characterized in that: The wavelength division multiplexer is a polarization-maintaining wavelength division multiplexer.

8. The figure-8 cavity mode-locked neodymium-doped femtosecond fiber laser according to claim 1, characterized in that: The 3×3 fiber coupler divides the light into two paths. The clockwise transmitted light is first amplified by the neodymium-doped fiber and then accumulates nonlinear phase shift in the nonlinear amplifying ring mirror. The light transmitted counterclockwise first accumulates nonlinear phase shift and finally is amplified by the neodymium-doped optical fiber; The two paths of light accumulate different nonlinear phase shifts related to the pulse intensity and interfere at the 3×3 fiber coupler. The center of the pulse with higher energy is transmitted while the two edges of the pulse with lower energy are reflected, thereby achieving mode locking.

9. The figure-8 cavity mode-locked neodymium-doped femtosecond fiber laser according to claim 8, characterized in that: The two beams of light converge and interfere at the 3×3 fiber coupler and then enter the linear end and the main ring; the fiber isolator in the main ring allows the input light to be transmitted unidirectionally in the main ring back to the 3×3 fiber coupler; the chirped fiber Bragg grating at the linear end reflects the light back to the 3×3 fiber coupler and compensates for the dispersion to further narrow the pulse; the laser reflected and unidirectionally transmitted back to the 3×3 fiber coupler enters the nonlinear amplifying ring mirror again, and thus reciprocates, the laser transmits and oscillates in the resonant cavity, and is finally output through the fiber output end of the 3×3 fiber coupler.

10. The figure-8 cavity mode-locked neodymium-doped femtosecond fiber laser according to claim 1, characterized in that: The dispersion parameter of the chirped fiber Bragg grating can be selected to compensate for the total dispersion of the entire resonant cavity.

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