A single-longitudinal-mode fiber laser

Abstract

The application provides a single-longitudinal-mode fiber laser which comprises a main cavity and a passive eye-shaped composite ring cavity filter module. The passive eye-shaped composite ring cavity filter module is required to select a single longitudinal mode from dense longitudinal modes of the main cavity, and the effective passband 3dB bandwidth of the eye-shaped composite ring cavity filter module must be 1-1.5 times of the longitudinal mode interval of the main cavity, and the effective free spectral range must be 0.5-1 times of the wavelength selection channel bandwidth, so that the single longitudinal mode laser output can be realized under the wavelength switching of the laser.

Description

Technical Field

[0001] This invention relates to the field of infrared laser technology, and more particularly to a switchable single-longitudinal-mode fiber laser. Background Technology

[0002] Single-wavelength / multi-wavelength switchable / continuously tunable single-longitudinal-mode fiber lasers occupy an extremely important position in the entire field of fiber lasers. Besides possessing all the advantages of ordinary fiber lasers, they also exhibit excellent coherence and monochromaticity. These lasers are generally used as high-quality coherent light sources or high-quality seed sources for further injection into subsequent fiber amplification stages to achieve high-power, narrow-linewidth output. The 2μm band single-longitudinal-mode thulium-doped fiber laser's eye safety, strong absorption by gas and water molecules, and high atmospheric transmittance give it not only the general advantages of single-longitudinal-mode fiber lasers but also unparalleled application advantages compared to other bands. It is crucial in industrial, scientific research, and civilian fields, with main applications including coherent optical communication, coherent Doppler lidar, ultra-long-distance fiber optic sensing, and optical wireless communication systems.

[0003] Based on the cavity length of the fiber laser, the cavity structure of fiber lasers achieving single-longitudinal-mode output is generally divided into two types: short-cavity structures, including distributed feedback fiber lasers and distributed Bragg reflector fiber lasers; and long-cavity structures with appropriate mode selection mechanisms. To date, both short-cavity and long-cavity structures of single-longitudinal-mode thulium-doped fiber lasers have been extensively reported, but most 2μm-band thulium-doped fiber lasers operate primarily in the 1900-2000nm region, which is the high-gain region of thulium-doped fibers. Reports on single-longitudinal-mode thulium-doped fiber lasers for longer wavelengths, such as the 2050nm band, are scarce. Since the 2050nm laser wavelength falls within the low-gain region of thulium-doped fibers, constructing a short-cavity 2050nm band single-longitudinal-mode thulium-doped fiber laser is extremely challenging. Besides replacing thulium-doped fiber with holmium-doped fiber, a long-cavity structure is needed to extend the length of the thulium-doped fiber within the cavity to provide sufficient gain in the 2050nm band. This, combined with other mode selection mechanisms, enables single-longitudinal-mode output in the 2050nm band. Comparing several long-cavity single-longitudinal-mode selection schemes, the ring-shaped composite cavity structure is more advantageous. The composite sub-ring, constructed from fiber couplers, is low-cost, easy to implement, and highly flexible, making it more suitable for the 2μm band, where optimized devices are lacking. Furthermore, the composite ring-shaped cavity structure, combined with other multi-wavelength channel filters, enables a multi-wavelength switchable 2050nm thulium-doped fiber laser where each channel operates in a single longitudinal mode, making it highly promising for next-generation lidar and wavelength division multiplexing free-space optical communication.

[0004] Thulium-doped fiber exhibits low gain around 2050 nm, making the construction of single-longitudinal-mode thulium-doped fiber lasers in this band challenging. Existing single-longitudinal-mode lasers in the 2050 nm band primarily employ short-cavity structures with thulium-holmium co-doped fiber as the gain fiber, or are commercially available distributed feedback semiconductor lasers; other laser structures are rarely reported. Increasing the length of the thulium-doped fiber using a long-cavity structure to provide sufficient gain is one solution. Compared to short-cavity single-longitudinal-mode lasers, long-cavity structures offer greater flexibility and performance scalability due to the inclusion of tuning devices. However, long-cavity structures result in smaller longitudinal-mode spacing, requiring reliable mechanisms to suppress multi-longitudinal-mode oscillations, such as using ultra-narrowband filters for longitudinal-mode selection. However, compared to ytterbium-doped and erbium-doped fiber lasers in the 1 μm and 1.5 μm bands, the construction of single-longitudinal-mode fiber lasers in the 2050 nm band is currently limited by a lack of optimized fiber components and effective measurement methods. Furthermore, compared to the 1μm and 1.5μm bands, 2μm lasing is more sensitive to intracavity losses. While using low-transmittance filters within the cavity of a single-mode ytterbium-doped fiber laser or a single-mode erbium-doped fiber laser may not significantly affect its output performance, it is extremely detrimental to a single-mode thulium-doped fiber laser because light in the 2μm band has higher transmission losses. This large intracavity loss introduced by low-transmittance filters can lead to unstable output laser light from a single-mode thulium-doped fiber laser, or even prevent laser generation altogether. Therefore, for single-mode thulium-doped fiber lasers, it is essential to increase the transmittance of the filters used and avoid introducing other losses. Summary of the Invention

[0005] The technical problem this invention aims to solve is how to improve the transmittance of the filter used in a single-mode thulium-doped fiber laser while avoiding the introduction of other losses. In view of this, this invention provides a switchable single-mode fiber laser.

[0006] The technical solution adopted in this invention is that the switchable single-longitudinal-mode fiber laser comprises:

[0007] Main cavity, passive eye-type composite ring cavity filter module;

[0008] The main cavity includes: a pump source, an optical fiber combiner, a thulium-doped fiber, an optical fiber isolator, an optical fiber circulator, a squeezed polarization controller, a sampling fiber Bragg grating, a polarizer, an optical fiber coupler, and an output port of the fiber laser.

[0009] The pump source is connected to the pump input port of the combiner, the signal arm output port of the combiner is connected to the thulium-doped fiber, the output of the thulium-doped fiber is connected to the input arm of the fiber isolator, the output of the fiber isolator is connected to the first input port of the fiber circulator, the second output port of the fiber circulator is connected to the output port of the squeeze polarization controller, the output port of the squeeze polarization controller is connected to the sampling fiber Bragg grating, the third output port of the fiber circulator is connected to the input port of the fiber polarizer, the output port of the fiber polarizer is connected to the input port of the passive eye-type composite cavity filter, the output port of the passive eye-type composite cavity filter is connected to the input port of the fiber coupler, the output arm of the fiber coupler is connected to the signal input fiber of the fiber combiner, and the output arm of the fiber coupler serves as the output end of a switchable single-longitudinal-mode fiber laser.

[0010] In one embodiment, the passive eye-type composite ring cavity filter module includes:

[0011] First fiber optic coupler, second fiber optic coupler, third fiber optic coupler, fourth fiber optic coupler, fiber optic input port, fiber optic output port;

[0012] Wherein, the first port of the first fiber optic coupler is connected to the fiber optic input port, the third port of the first fiber optic coupler is unused, the fourth port of the first fiber optic coupler is connected to the fifth port of the second fiber optic coupler, the seventh port of the second fiber optic coupler is connected to the tenth port of the third fiber optic coupler, the eighth port of the second fiber optic coupler is connected to the thirteenth port of the fourth fiber optic coupler, the ninth port of the third fiber optic coupler is unused, the eleventh port of the third fiber optic coupler is connected to the fourteenth port of the fourth fiber optic coupler, the fifteenth port of the fourth fiber optic coupler is connected to the sixth port of the second fiber optic coupler, the sixteenth port of the fourth fiber optic coupler is connected to the second port of the first fiber optic coupler, and the twelfth port of the third fiber optic coupler is connected to the fiber optic output end.

[0013] In one embodiment, the first, second, third, and fourth fiber optic couplers are all 2x2 fiber optic couplers, with splitting ratios of 70:30, 99.99:0.01, 99.99:0.01, and 70:30, respectively.

[0014] By adopting the above technical solution, the present invention has at least the following advantages:

[0015] The switchable single-longitudinal-mode fiber laser of this invention requires a passive eye-type composite cavity filter module to select a single longitudinal mode from the dense longitudinal modes of the main cavity. This places requirements on the eye-type composite cavity filter module: its effective passband 3dB bandwidth must be 1 to 1.5 times the longitudinal mode spacing of the main cavity, and its effective free spectral range must be 0.5 to 1 times the wavelength selection channel bandwidth. In this way, when the laser is assembled, single-longitudinal-mode laser output can be achieved under arbitrary wavelength switching. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of a switchable single-longitudinal-mode fiber laser according to an embodiment of the present invention.

[0017] Figure 2 A diagram showing the parameters of the sampling fiber Bragg grating used in the main cavity according to an embodiment of the present invention;

[0018] Figure 3 This is the output transmission spectrum of the eye-type composite ring cavity filter module according to an embodiment of the present invention.

[0019] Figure Labels

[0020] Pump source 101, fiber combiner 102, thulium-doped fiber 103, fiber isolator 104, fiber circulator 105, squeeze polarization controller 106, sampling fiber Bragg grating 107, polarizer 108, first fiber coupler 110, second fiber coupler 111, third fiber coupler 112, fourth fiber coupler 113, fiber input port 109, fiber output port 114, fiber coupler 115, fiber laser output port 116. Detailed Implementation

[0021] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the present invention will be described in detail below with reference to the accompanying drawings and preferred embodiments.

[0022] It should be understood that the terms "comprising," "including," "having," "containing," and / or "comprises," when used in this specification, indicate the presence of the stated features, integrals, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components, and / or combinations thereof. Furthermore, when expressions such as "at least one of..." appear after a list of listed features, they modify the entire listed feature, not individual elements in the list. Additionally, when describing embodiments of this application, the word "may" is used to mean "one or more embodiments of this application." And the term "exemplary" is intended to refer to an example or illustration.

[0023] As used herein, the terms “basically,” “approximately,” and similar terms are used as terms of approximation rather than terms of degree, and are intended to describe inherent biases in measured or calculated values ​​that will be recognized by those skilled in the art.

[0024] Unless otherwise specified, all terms used herein (including technical and scientific terms) shall have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. It should also be understood that terms (e.g., those defined in common dictionaries) shall be interpreted as having the meaning consistent with their meaning in the context of the relevant art and shall not be interpreted in an idealized or overly formal sense unless expressly so specified herein.

[0025] Embodiments of the present invention provide a switchable single-longitudinal-mode fiber laser, such as... Figure 1 As shown, the system includes a main cavity and a passive eye-type composite cavity filter module. The main cavity includes: a pump source 101, an optical fiber combiner 102, a thulium-doped fiber 103, an optical fiber isolator 104, an optical fiber circulator 105, a squeeze-type polarization controller 106, a sampling fiber Bragg grating 107, a polarizer 108, an optical fiber coupler 115, and an optical fiber laser output port 116. The passive eye-type composite cavity filter module includes: a first optical fiber coupler 110, a second optical fiber coupler 111, a third optical fiber coupler 112, a fourth optical fiber coupler 113, an optical fiber input port 109, and an optical fiber output port 114.

[0026] In the main cavity: the pump source 101 is connected to the pump input port of the combiner 102 to provide gain for the thulium-doped fiber. The signal arm output port of the combiner 102 is connected to the thulium-doped fiber 103. The output of the thulium-doped fiber 103 is connected to the input arm of the fiber isolator 104 to ensure unidirectional operation of the laser. The output of the fiber isolator 104 is connected to input port 1 of the fiber circulator 105. Output port 2 of the fiber circulator 105 is connected to the output port of the squeeze polarization controller 106. The output port of the squeeze polarization controller 106 is connected to the sampling fiber Bragg grating 107, and the light is reflected back from the sampling fiber Bragg grating 107. The optical signal at the corresponding wavelength is transmitted from port 2 of the fiber optic circulator 105 to port 3 of the fiber optic circulator 105. The output port 3 of the fiber optic circulator is connected to the input port of the fiber polarizer 108. The output port of the fiber polarizer 108 is connected to the input port 109 of the passive eye-type composite cavity filter. The output port 114 of the passive eye-type composite cavity filter is connected to the input port of the fiber coupler 115. The output arm 117 of the fiber coupler 115 is connected to the signal input fiber of the fiber combiner 102. The output arm 116 of the fiber coupler 115 serves as the output end of the three-wavelength switchable single longitudinal mode fiber laser.

[0027] Furthermore, the squeeze-type polarization controller 106, the sampling fiber Bragg grating 107, and the fiber polarizer 108 together constitute polarization-dependent loss modulation of the laser cavity. By adjusting the squeeze-type polarization controller 106, the individual reflection wavelengths of the sampling fiber Bragg grating 107 can be switched individually. When spontaneously emitted light with arbitrary polarization generated by the gain fiber passes through port 2 of the fiber circulator 105, the polarization state of the light is changed by the squeeze-type polarization controller 106, and then reflected at various wavelengths of the sampling fiber Bragg grating 107. The parameters of each reflection wavelength are as follows: Figure 2 As shown, the polarization state of the reflected light is rotated again by the adjustable squeeze polarization controller 106 before the sampling fiber Bragg grating 107, and then injected into the subsequently connected fiber polarizer 108 after passing through the fiber circulator 2. Since the fiber polarizer 108 only allows light of a specific linear polarization state to pass through, different polarization-dependent losses will occur when light of different polarization states passes through the fiber polarizer 108. According to the laser oscillation conditions, the wavelength with the lowest single-loop transmission loss can be lased. Here, the total loss for each wavelength includes cavity loss and polarization-dependent loss. Based on the gain competition effect of the uniformly broadened rare-earth-doped fiber, the polarization-dependent loss is adjusted by regulating the adjustable squeeze polarization controller 106, and finally, a switchable laser between the three wavelengths determined by the sampling fiber Bragg grating 107 can be obtained.

[0028] In the passive eye-type composite cavity filter module, the first port of the first fiber coupler 110 is connected to the fiber input port 109, the third port of the first fiber coupler 110 is unused, the fourth port of the first fiber coupler 110 is connected to the fifth port of the second fiber coupler 111, the seventh port of the second fiber coupler 111 is connected to the tenth port of the third fiber coupler 112, the eighth port of the second fiber coupler 111 is connected to the thirteenth port of the fourth fiber coupler 113, the ninth port of the third fiber coupler 112 is unused, the eleventh port of the third fiber coupler 112 is connected to the fourteenth port of the fourth fiber coupler 113, the fifteenth port of the fourth fiber coupler 113 is connected to the sixth port of the second fiber coupler 111, the sixteenth port of the fourth fiber coupler 113 is connected to the second port of the first fiber coupler 110, and the twelfth port of the third fiber coupler 112 is connected to the fiber output terminal 114.

[0029] Preferably, the first fiber coupler 110, the second fiber coupler 111, the third fiber coupler 112, and the fourth fiber coupler 113 are all 2X2 fiber couplers with splitting ratios of 70:30, 99.99:0.01, 99.99:0.01, and 70:30, respectively.

[0030] In this invention, the passive eye-type composite cavity filter module needs to select a single longitudinal mode from the dense longitudinal modes generated by the main cavity. This places requirements on the passive eye-type composite cavity filter module: its effective passband 3dB bandwidth must be 1 to 1.5 times the longitudinal mode spacing of the main cavity, and its effective free spectral range must be 0.5 to 1 times the wavelength selection channel bandwidth. In this way, when the laser is assembled, single longitudinal mode laser output can be achieved under arbitrary wavelength switching.

[0031] according to Figure 2 The sampling fiber Bragg grating 107 provides the bandwidth of each reflection peak, and the effective free spectral range (FSR) of the transmission passband formed by the passive eye-type composite cavity filter module. eff The effective free spectral range (FSR) of the passive eye composite cavity filter should be 0.5-1 times the reflection bandwidth of the sampling fiber Bragg grating 107. Therefore, only one effective transmission passband of the passive eye composite cavity filter dominates within each reflection passband of the sampling fiber Bragg grating 107. The effective free spectral range of the passive eye composite cavity filter is determined by the free spectral range (FSR) of each sub-cavity within the composite cavity, FSR = c / (nL), where c is the speed of light, n is the refractive index, and L is the cavity length of each sub-cavity. Based on the vernier effect, the FSR of the passive eye composite cavity filter... eff This is the least common multiple of the free spectral ranges of each sub-cavity. Figure 2 It can be seen that the maximum 3dB transmission bandwidth at the reflection wavelength of the sampling fiber Bragg grating 107 is 0.16nm. Figure 3 The FSR of the passive eye-type composite cavity filter is visible. eff The wavelength is 0.135nm, which meets the requirement of 0.5-1 times the reflection bandwidth.

[0032] This invention designs an integrated passive eye-shaped composite annular cavity with an active annular main cavity of length L0. The longitudinal mode spacing corresponding to the main cavity is Δν. q = c / (nL0), where c is the speed of light and n is the refractive index of the single-mode fiber core. Therefore, the 3dB bandwidth Δν of the main transmission peak within an effective free spectral range of the integrated passive eye-type composite cavity filter is... c It must be 1-1.5 times the longitudinal mold spacing of the main cavity, i.e., Δν c = (1~1.5)×Δν q The formula for calculating the spectral linewidth of a passive composite ring cavity is: Δν c =cδ / (2πL) c ), L cδ represents the fiber length of the longest subcavity in the passive eye-type composite cavity, and δ represents the single-pass loss of the passive eye-type composite cavity, including the coupling splitting ratio loss of the first fiber coupler 110, the second fiber coupler 111, the third fiber coupler 112, and the fourth fiber coupler 113, as well as the fusion loss at each fusion point. Generally, the cavity length of the active main cavity of a laser is around 10-15m, corresponding to a longitudinal mode spacing of 20.8MHz-13.9MHz. The spectral linewidth of the eye-type composite cavity filter module proposed in this invention is 26.38MHz. Figure 3 As shown, this spectral width meets the requirement of 1-1.5 times the longitudinal mode spacing of the main cavity.

[0033] According to the above design, the passive eye-type composite cavity filter proposed in this embodiment can meet the requirements of single longitudinal mode filtering of the main cavity and realize three-wavelength switchable single longitudinal mode laser output based on the passive eye-type composite cavity filter in the 2050nm band.

[0034] In summary, compared with the prior art, the present invention has at least the following advantages:

[0035] 1) This embodiment uses a sampling fiber Bragg grating to provide a multi-wavelength selection channel for the 2050nm band.

[0036] 2) This embodiment can realize three-wavelength switchable single longitudinal mode laser output in the 2050nm band.

[0037] 3) In this embodiment, the passive eye-type composite ring cavity filter module needs to select a single longitudinal mode from the dense longitudinal modes of the main cavity. This places requirements on the eye-type composite ring cavity filter module: its effective passband 3dB bandwidth must be 1 to 1.5 times the longitudinal mode spacing of the main cavity, and its effective free spectrum range must be 0.5 to 1 times the wavelength selection channel bandwidth. In this way, when the laser is assembled, single longitudinal mode laser output can be achieved under arbitrary wavelength switching.

[0038] 4) The laser structure of this embodiment has the advantages of low price and high flexibility. By optimizing the length of each sub-cavity in the eye-shaped composite ring cavity filter module and the coupling ratio of the fiber coupler, different filtering effects can be achieved. It is especially suitable for single longitudinal mode filtering of 2050nm band fiber lasers where key filtering components are relatively lacking.

[0039] Through the description of specific embodiments, a more in-depth and specific understanding should be gained of the technical means and effects adopted by the present invention to achieve the intended purpose. However, the accompanying drawings are only provided for reference and illustration and are not intended to limit the present invention.

Claims

1. A switchable single-longitudinal-mode fiber laser, characterized in that, include: Main cavity, passive eye-type composite ring cavity filter module; The main cavity includes: a pump source, an optical fiber combiner, a thulium-doped fiber, an optical fiber isolator, an optical fiber circulator, a squeezed polarization controller, a sampling fiber Bragg grating, an optical fiber polarizer, an optical fiber coupler, and an output port of the optical fiber laser. The pump source is connected to the pump input port of the fiber combiner, the signal arm output port of the fiber combiner is connected to the thulium-doped fiber, the output of the thulium-doped fiber is connected to the input arm of the fiber isolator, the output of the fiber isolator is connected to the first input port of the fiber circulator, the second output port of the fiber circulator is connected to the output port of the squeeze polarization controller, the output port of the squeeze polarization controller is connected to the sampling fiber Bragg grating, the third output port of the fiber circulator is connected to the input port of the fiber polarizer, the output port of the fiber polarizer is connected to the input port of the passive eye-type composite cavity filter, the output port of the passive eye-type composite cavity filter is connected to the input port of the fiber coupler, the output arm of the fiber coupler is connected to the signal input fiber of the fiber combiner, and the output arm of the fiber coupler serves as the output end of a switchable single-longitudinal-mode fiber laser. The passive eye-type composite ring cavity filter module includes: First fiber optic coupler, second fiber optic coupler, third fiber optic coupler, fourth fiber optic coupler, fiber optic input port, fiber optic output port; Wherein, the first port of the first fiber optic coupler is connected to the fiber optic input port, the third port of the first fiber optic coupler is unused, the fourth port of the first fiber optic coupler is connected to the fifth port of the second fiber optic coupler, the seventh port of the second fiber optic coupler is connected to the tenth port of the third fiber optic coupler, the eighth port of the second fiber optic coupler is connected to the thirteenth port of the fourth fiber optic coupler, the ninth port of the third fiber optic coupler is unused, the eleventh port of the third fiber optic coupler is connected to the fourteenth port of the fourth fiber optic coupler, the fifteenth port of the fourth fiber optic coupler is connected to the sixth port of the second fiber optic coupler, the sixteenth port of the fourth fiber optic coupler is connected to the second port of the first fiber optic coupler, and the twelfth port of the third fiber optic coupler is connected to the fiber optic output end.

2. The switchable single-longitudinal-mode fiber laser according to claim 1, characterized in that, The first, second, third, and fourth fiber optic couplers are all 2x2 fiber optic couplers, with splitting ratios of 70:30, 99.99:0.01, 99.99:0.01, and 70:30, respectively.

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