A wavelength-switchable tunable dual-wavelength single-frequency fiber laser

Abstract

The application discloses a wavelength-switchable and tunable dual-wavelength single-frequency fiber laser based on birefringence effect, which comprises a partially reflective polarization maintaining fiber grating, a gain fiber, a wavelength division multiplexer, a pump source, a 3dB fiber coupler and an unpumped active fiber; the partially reflective polarization maintaining fiber grating is used as a reflective cavity mirror and simultaneously realizes laser output; the birefringence effect of the fiber is utilized to generate dual-wavelength laser, and wavelength tuning and wavelength switching are realized under the action of longitudinal stress; the 3dB fiber coupler and the unpumped active fiber form a Sagnac loop reflector, which provides feedback for the laser and realizes single longitudinal mode operation as a frequency selection device; the gain fiber absorbs the pump laser injected by the pump source to form particle number inversion and provide gain for the laser. The application utilizes the unpumped active fiber as a saturated absorber for frequency selection and combines the birefringence effect of the partially reflective polarization maintaining fiber grating to realize watt-level dual-wavelength single-frequency laser.

Description

Technical Field

[0001] This invention relates to the field of single-frequency fiber lasers, and more particularly to a wavelength-switchable and tunable dual-wavelength single-frequency fiber laser based on birefringence. Background Technology

[0002] Compared to single-wavelength single-frequency fiber lasers, dual-wavelength single-frequency fiber lasers offer advantages in gas sensing systems, including multi-parameter detection and distributed fiber sensing. Based on active fibers with different rare-earth doping and wavelength selection techniques, dual-wavelength single-frequency fiber laser outputs operating in the 1μm, 1.5μm, and 2μm bands have been achieved. To achieve single-frequency operation of dual-wavelength lasers, the laser resonator must first employ a special frequency selection method, typically using ultrashort cavities, cascaded subcavities, or saturable absorbers. Ultrashort cavity methods generally combine grating arrays, grating groups, or polarization aperture burning effects based on polarization-maintaining fiber gratings to achieve dual-wavelength single-frequency fiber laser output. While ultrashort cavity methods can achieve relatively stable dual-wavelength laser output, the shorter active fiber length limits the increase in laser output power. High-precision subcavity or saturable absorber methods are often used in fiber optic oscillators with ring cavity structures. Their long cavity lengths allow for a wide range of wavelength selection methods, such as fiber grating arrays, FP cavities, Mach-Zehnder interferometers, cascaded Sagnac rings, multimode interferometers, or polarization-maintaining fiber gratings. However, the presence of multiple subcavities increases the complexity of the resonant cavity and significantly challenges the stability of single-frequency laser operation, especially at high pump powers. Even the highest dual-wavelength single-frequency output power is only in the tens of milliwatts range. Saturable absorber frequency selection methods are widely used in dual-wavelength single-frequency fiber lasers with linear and ring cavity structures. Combined with the aforementioned dual-wavelength filters, dual-wavelength laser oscillation is achieved. The spatial hole-burning effect in the unpumped active fiber is used to create a narrowband refractive index grating with two transmission wavelengths, simultaneously satisfying the single-frequency operation conditions for dual-wavelength lasers. This is an excellent frequency selection method suitable for dual-wavelength single-frequency operation. Although dual-wavelength single-frequency fiber lasers have been widely reported, their output power is generally low, with the highest being only in the hundreds of milliwatts range, and it is difficult to achieve wavelength tuning and switching of dual-wavelength lasers simultaneously.

[0003] In summary, although dual-wavelength single-frequency lasers have been widely reported, achieving high output power and wavelength tuning and switching remain technical challenges in this field. Furthermore, current dual-wavelength single-frequency fiber lasers struggle to simultaneously achieve wavelength switching and tuning. Based on the birefringence effect of polarization-maintaining fiber, the center reflection wavelengths of the obtained fiber grating are located on the fast and slow axes of the polarization-maintaining fiber. Using this as a wavelength selection device for the laser cavity, combined with a saturable absorber as an ultra-narrowband filter, is a simple way to realize dual-wavelength single-frequency fiber lasers. Moreover, the polarization-maintaining fiber grating modulates the center wavelength and reflectivity of its reflection spectrum under longitudinal tension, which provides the possibility for wavelength switching and tuning of dual-wavelength lasers. Summary of the Invention

[0004] This invention provides a wavelength-switchable and tunable dual-wavelength single-frequency fiber laser. The invention utilizes the birefringence effect of a polarization-maintaining fiber grating to achieve dual-wavelength laser operation. Switching and tuning of the dual-wavelength laser are achieved by applying radial tension to the fiber grating. Furthermore, a saturable absorber is used as a frequency-selective device to achieve single-longitudinal-mode laser operation. See the description below for details:

[0005] A wavelength-switchable and tunable dual-wavelength single-frequency fiber laser, the laser comprising: a partially reflective polarization-maintaining fiber grating, a gain fiber, a wavelength division multiplexer, a pump source, a 3dB fiber coupler, and an unpumped active fiber.

[0006] The partially reflective polarization-maintaining fiber grating serves as a reflective cavity mirror while simultaneously enabling laser output. It utilizes the birefringence effect of the fiber to generate dual-wavelength lasers and achieves wavelength tuning and switching under longitudinal stress.

[0007] A 3dB fiber coupler and an unpumped active fiber form a Sagnac ring reflector, which provides feedback to the laser and acts as a frequency-selective device to achieve single-longitudinal-mode operation.

[0008] The partially reflective polarization-maintaining fiber grating includes two reflection bands: a short-wavelength reflection band corresponding to the fast axis and a long-wavelength reflection band corresponding to the slow axis. The two bands oscillate simultaneously in the laser resonator and are orthogonally linearly polarized lasers.

[0009] Furthermore, the wavelength tuning is as follows:

[0010] Radial stress is applied to a partially reflective polarization-maintaining fiber grating to change the refractive index modulation period, thereby changing the center wavelength of the reflection band. During laser oscillation, the laser wavelength is tuned by changing the radial tension applied to the partially reflective polarization-maintaining fiber grating.

[0011] Wherein, the wavelength switching is:

[0012] Applying radial tension to a partially reflective polarization-maintaining fiber grating changes the center wavelength of the grating's reflection band, thus achieving laser wavelength tuning; changing the reflectivity of the short-wavelength and long-wavelength reflection bands allows for laser wavelength switching.

[0013] Furthermore, the dual-wavelength laser forms a refractive index modulation grating with two narrow transmission bands within the saturable absorber, simultaneously satisfying the frequency selectivity of the two wavelengths.

[0014] The center wavelength of the partially reflective polarization-maintaining fiber grating is any wavelength from near-infrared to mid-infrared. The doping ions of the gain fiber are ytterbium ions, thulium ions, erbium ions, or holmium ions.

[0015] The doped ions in the unpumped active optical fiber are ytterbium, thulium, erbium, or holmium ions. The pump source wavelength satisfies the absorption conditions of the gain fiber.

[0016] The beneficial effects of the technical solution provided by this invention are:

[0017] (1) This invention utilizes the birefringence phenomenon to achieve dual-wavelength lasing, which has strong applicability in the near-infrared to mid-infrared wavelength range; applying longitudinal stress to the fiber grating can achieve wavelength switching and tuning without the need for additional optical devices.

[0018] (2) The present invention adopts a linear cavity structure. By increasing the length of the active optical fiber, sufficient laser gain can be guaranteed to achieve high-power laser output.

[0019] (3) This invention utilizes a saturable absorber to achieve frequency selection, enabling single-longitudinal-mode operation in dual-wavelength and single-wavelength states, and achieving effective frequency selection within the laser wavelength tuning range, thus realizing tunable single-frequency laser operation.

[0020] (4) The present invention can simultaneously meet the needs of dual-wavelength, single-wavelength, wavelength tuning and wavelength switching operation states by using only fiber optic gratings. The method is simple and greatly simplifies the structure of the laser system. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of a wavelength-switchable and tunable dual-wavelength single-frequency fiber laser.

[0022] The attached diagram lists the components represented by each number as follows:

[0023] 1: Unpumped active fiber; 2: 3dB 2×1 fiber coupler;

[0024] 3: Gain fiber; 4: Polarization-maintaining output fiber grating;

[0025] 5: Wavelength division multiplexer; 6: Pump source;

[0026] 7(1): Left fiber optic grating clamping and stress application device;

[0027] 7(2): Right side fiber grating clamping and stress application device. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below.

[0029] Dual-wavelength laser operation is achieved by utilizing the birefringence effect of polarization-maintaining fiber gratings. Switching and tuning of the dual-wavelength laser are realized by applying radial tension to the fiber grating. Furthermore, a saturable absorber is used as a frequency-selective device to achieve single-longitudinal-mode operation of the laser. The specific scheme is as follows:

[0030] A wavelength-switchable and tunable dual-wavelength single-frequency fiber laser consists of a partially reflective polarization-maintaining fiber grating 4, a gain fiber 3, a wavelength division multiplexer 5, a pump source 6, a 3dB fiber coupler 2, and an unpumped active fiber 1.

[0031] Among them, the partially reflective polarization-maintaining fiber grating 4 serves as a reflective cavity mirror to achieve laser output, and on the other hand, it utilizes the birefringence effect of the fiber to generate dual-wavelength laser, and achieves wavelength tuning and wavelength switching under longitudinal stress.

[0032] In this embodiment, a 3dB 2×1 fiber coupler 2 and an unpumped active fiber 1 form a Sagnac ring reflector, which provides feedback to the laser and serves as a frequency-selective device to achieve single-mode operation. The Sagnac ring reflector is a well-known technical term in the art, and will not be described in detail in this embodiment.

[0033] In this process, the gain fiber 3 absorbs the pump laser injected by the pump source 6, forming a population inversion, thereby providing gain to the laser.

[0034] The principle of dual-wavelength laser generation is as follows: the birefringence phenomenon in the partially reflective polarization-maintaining fiber grating 4 gives it two reflection bands, with the short-wavelength reflection band corresponding to the fast axis and the long-wavelength reflection band corresponding to the slow axis. The two bands oscillate simultaneously in the laser resonator and are orthogonal linearly polarized lasers.

[0035] Furthermore, the tuning principle of dual-wavelength laser is as follows: By applying radial stress to the partially reflective polarization-maintaining fiber grating 4, its refractive index modulation period can be changed, thereby altering the center wavelength of the reflection band. During laser oscillation, the laser wavelength can be tuned by changing the radial tension applied to the partially reflective polarization-maintaining fiber grating 4.

[0036] Furthermore, the wavelength switching principle of dual-wavelength laser is as follows: By applying radial tension to the partially reflective polarization-maintaining fiber grating 4, the center wavelength of the grating reflection band can be changed to achieve laser wavelength tuning; on the other hand, by changing the reflectivity of the short-wavelength and long-wavelength reflection bands, the gain competition state of the dual-wavelength laser can be affected, thereby achieving laser wavelength switching.

[0037] Furthermore, the dual-wavelength laser single-longitudinal-mode operation is achieved by using an unpumped active fiber 1 and a 3dB 2×1 fiber coupler 2 to form a saturable absorber for frequency selection. The dual-wavelength laser forms a refractive index modulation grating with two narrow transmission bands within the saturable absorber, simultaneously satisfying the frequency selection of both wavelengths.

[0038] The center wavelength of the partially reflective polarization-maintaining fiber grating 4 can be any wavelength from near-infrared to mid-infrared.

[0039] Among them, the doping ions of the gain fiber 3 can be ytterbium ions, thulium ions, erbium ions, holmium ions, etc., as long as the center wavelength lasing condition of the partially reflective fiber grating 4 is met.

[0040] Furthermore, the doped ions of the unpumped active fiber 1 can be ytterbium ions, thulium ions, erbium ions, holmium ions, etc., as long as they can achieve a saturable absorption effect on the center wavelength laser of the partially reflected polarization-maintaining fiber grating 4.

[0041] Furthermore, the pump source wavelength only needs to satisfy the absorption condition of gain fiber 3.

[0042] Example 1

[0043] A wavelength-switchable and tunable dual-wavelength single-frequency fiber laser is disclosed. The laser is based on the birefringence effect. The unpumped active fiber 1 is preferably an 11m thulium-holmium co-doped fiber. The 3dB 2×1 fiber coupler 2 is preferably a 50 / 50 2050nm fiber coupler. The gain fiber 3 is preferably a 10m thulium-doped fiber. The partially reflective polarization-maintaining fiber grating 4 is preferably a polarization-maintaining fiber grating with a center wavelength of 2050nm, a reflectivity of 50%, and a reflection bandwidth of 0.07nm, etched on a PM1550 fiber. The wavelength division multiplexer 5 is preferably a 1570 / 2050nm filtered wavelength division multiplexer. The pump source 6 is preferably a 1570nm fiber laser.

[0044] Among them, the left fiber grating clamping and stress application device 7(1) and the right fiber grating clamping and stress application device 7(2) are used to fix the partial reflection polarization-maintaining fiber grating 4 and apply longitudinal stress.

[0045] In a specific implementation, the stress application device can be a three-dimensional adjustment frame, on which an optical fiber holding device is fixed. Radial tension is applied to the partially reflective polarization-maintaining fiber grating 4 by adjusting the forward and backward movement of the three-dimensional adjustment frame using a knob. The specific implementation and structure of the stress application device in this embodiment are not limited, as long as it can hold the optical fiber and adjust the tension.

[0046] The pump source 6 has an output power of 5W. Without longitudinal tension, the partially reflective polarization-maintaining fiber grating 4 can achieve single-frequency operation with a dual-wavelength output power of 1W. By applying radial tension to the partially reflective fiber grating 4, wavelength tuning and wavelength switching can be achieved simultaneously, with a wavelength tuning range of 5nm. During wavelength switching, the laser output power does not change significantly.

[0047] In the above embodiments, the center wavelength of the partially reflective polarization-maintaining fiber grating 4 can be any wavelength from near-infrared to mid-infrared; the doping ions of the gain fiber 3 can be ytterbium ions, thulium ions, erbium ions, holmium ions, etc., as long as the lasing condition of the center wavelength of the partially reflective polarization-maintaining fiber grating 4 is met.

[0048] In the above embodiments, the doped ions of the unpumped active fiber 1 can be ytterbium ions, thulium ions, erbium ions, holmium ions, etc. The wavelength of the pump source 6 only needs to meet the absorption conditions of the unpumped active fiber 1 for the pump laser, and the wavelength of the pump source 6 only needs to meet the absorption conditions of the gain fiber 3.

[0049] In the above embodiments, the fiber grating clamping and stress application devices 7(1) and 7(2) only need to apply an effective radial tension to the partially reflective polarization-maintaining fiber grating 4 without damaging the physical structure of the fiber.

[0050] Among them, the partially reflective polarization-maintaining fiber grating 4 serves as a feedback cavity mirror to realize laser output on the one hand, and provides birefringence effect to generate dual-wavelength laser on the other hand; wavelength tuning and wavelength switching control are achieved by applying radial tension to the partially reflective fiber grating 4.

[0051] A resonant cavity composed of a Sagnac ring reflector and a partially reflective polarization-maintaining fiber grating 4 is used to achieve simultaneous oscillation of dual-wavelength lasers within the resonant cavity, with the two being orthogonally linearly polarized lasers.

[0052] By applying a radial tension to the partially reflective polarization-maintaining fiber grating 4, its refractive index modulation period can be changed, thereby altering the center wavelength of the reflection band. During laser oscillation, the lasing wavelength can be tuned by changing the radial tension applied to the partially reflective polarization-maintaining fiber grating 4.

[0053] The center wavelength of the partially reflective fiber grating 4 can be any wavelength from near-infrared to mid-infrared.

[0054] Furthermore, the doping ions of the gain fiber 3 can be ytterbium ions, thulium ions, erbium ions, holmium ions, etc., as long as they meet the lasing condition of the center wavelength of the partially reflective fiber grating 4.

[0055] The doped ions of the unpumped active fiber 1 can be ytterbium ions, thulium ions, erbium ions, holmium ions, etc., as long as they have saturable absorption characteristics for lasers located at the center wavelength of the partially reflective fiber grating 4.

[0056] Furthermore, the wavelength of pump source 6 needs to be located within the rare-earth ion absorption band of the gain fiber 3.

[0057] Unless otherwise specified, the model numbers of the various devices in this embodiment of the invention are not limited, and any device that can perform the above functions is acceptable.

[0058] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of a preferred embodiment, and the sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0059] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A wavelength-switchable and tunable dual-wavelength single-frequency fiber laser, characterized in that, The laser includes: a partially reflective polarization-maintaining fiber grating, a gain fiber, a wavelength division multiplexer, a pump source, a 3dB fiber coupler, and an unpumped active fiber. The partially reflective polarization-maintaining fiber grating serves as a reflective cavity mirror while simultaneously enabling laser output. It utilizes the birefringence effect of the fiber to generate dual-wavelength lasers and achieves wavelength tuning and switching under longitudinal stress. A 3dB fiber coupler and an unpumped active fiber form a Sagnac ring reflector, which provides feedback to the laser and acts as a frequency-selective device to achieve single-longitudinal-mode operation. Among them, the partially reflective polarization-maintaining fiber grating includes two reflection bands: the short-wavelength reflection band corresponds to the fast axis, and the long-wavelength reflection band corresponds to the slow axis. The two bands oscillate simultaneously in the laser resonator and are orthogonal linearly polarized lasers. Wherein, the wavelength tuning is: Radial stress is applied to a partially reflective polarization-maintaining fiber grating to change the refractive index modulation period, thereby changing the center wavelength of the reflection band; during laser oscillation, the laser wavelength is tuned by changing the radial tension applied to the partially reflective polarization-maintaining fiber grating. The wavelength switching is achieved by: applying radial tension to a partially reflective polarization-maintaining fiber grating to change the center wavelength of the grating's reflection band to achieve laser wavelength tuning; and changing the reflectivity of the short-wavelength and long-wavelength reflection bands to achieve laser wavelength switching.

2. The wavelength-switchable and tunable dual-wavelength single-frequency fiber laser according to claim 1, characterized in that, A dual-wavelength laser forms a refractive index modulation grating with two narrow transmission bands within a saturable absorber, simultaneously satisfying the frequency selectivity of both wavelengths.

3. A wavelength-switchable and tunable dual-wavelength single-frequency fiber laser according to claim 1, characterized in that, The center wavelength of the partially reflective polarization-maintaining fiber grating is any wavelength from near-infrared to mid-infrared.

4. A wavelength-switchable and tunable dual-wavelength single-frequency fiber laser according to claim 1, characterized in that, The doping ions of the gain fiber are ytterbium ions, thulium ions, erbium ions, or holmium ions.

5. A wavelength-switchable and tunable dual-wavelength single-frequency fiber laser according to claim 1, characterized in that, The doped ions of the unpumped active optical fiber are ytterbium ions, thulium ions, erbium ions, or holmium ions.

6. A wavelength-switchable and tunable dual-wavelength single-frequency fiber laser according to claim 1, characterized in that, The pump source wavelength satisfies the absorption conditions of the gain fiber.

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