CN110797737B - Short straight cavity single-polarization single-longitudinal mode optical fiber laser and preparation method thereof - Google Patents
Short straight cavity single-polarization single-longitudinal mode optical fiber laser and preparation method thereof Download PDFInfo
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
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Abstract
The invention discloses a short straight cavity single polarization single longitudinal mode fiber laser and a preparation method thereof, wherein the preparation method comprises the following steps: the active optical fiber is inscribed with a first fiber Bragg grating and a second fiber Bragg grating, the first fiber Bragg grating is used for forming a high-reflection cavity mirror, and the second fiber Bragg grating is used for forming an output cavity mirror; one end of the active optical fiber, on which the second fiber Bragg grating is engraved, is connected with the common end of the wavelength division multiplexer; the slow (fast) axis of the first fiber Bragg grating is parallel to the fast (slow) axis of the second fiber Bragg grating and corresponds to the same polarization mode, the Bragg reflection peak wavelength corresponding to the slow axis of the first fiber Bragg grating is equal to the Bragg reflection peak wavelength corresponding to the fast axis of the second fiber Bragg grating, and the polarization is separated from the Bragg reflection peak in the orthogonal direction. The invention can directly manufacture the short straight cavity single polarization single longitudinal mode fiber laser on the active fiber by using a femtosecond laser phase mask method.
Description
Technical Field
The invention belongs to the technical field of fiber optics and fiber lasers, and particularly relates to a short straight cavity single-polarization single-longitudinal mode fiber laser and a preparation method thereof.
Background
The single longitudinal mode fiber laser has the advantages of good stability, good beam quality, low threshold value, high efficiency and the like, and is widely applied to the fields of fiber sensing, fiber communication, laser ranging and the like. In various single longitudinal mode fiber laser schemes, the fiber laser adopting an ultra-short line cavity (ultra-short straight cavity) has the advantages of small volume and high integration level. To obtain a single longitudinal mode output, it is necessary to use as short a cavity length as possible. In addition to narrow linewidth, many applications such as nonlinear mixing, coherent combining require that the laser be single-polarized.
Generally, a short straight-cavity single-polarization fiber laser adopts a polarization-maintaining active fiber as a gain medium, and a high-reflection cavity mirror and a low-reflection cavity mirror adopt a polarization-maintaining fiber bragg grating and a non-polarization-maintaining broadband fiber bragg grating combination or a pair of polarization-maintaining fiber bragg gratings combination. Because the polarization-maintaining fiber Bragg grating is provided with two reflection peaks with two mutually orthogonal polarizations, only one pair of Bragg reflection peaks in the two gratings can be superposed through the parameter selection of the fiber Bragg grating, and thus, single-polarization output is realized. Because the fiber Bragg grating is difficult to prepare on the active fiber, the Bragg grating is usually engraved on the passive polarization-maintaining fiber, and then the fiber Bragg grating is fused with the active fiber to form the polarization-maintaining fiber laser. Since it is necessary to include some passive fiber, the cavity length of the laser is actually increased, reducing the gain of the laser. In addition, the polarization maintaining fiber is high in price compared with a passive fiber, and the directivity needs to be considered during fusion, so that the manufacturing difficulty is increased.
In recent years, a femtosecond laser can be used for directly writing a fiber Bragg grating on an active fiber to prepare a laser, so that fusion welding and a passive part in a cavity are avoided. The femtosecond laser writing of the fiber bragg grating comprises a direct writing method and a phase mask plate method, wherein the femtosecond laser and phase mask plate combined fiber bragg grating writing method has the advantages of good repeatability and high precision, and is very suitable for batch preparation of the fiber bragg grating. However, because the fiber bragg grating etched by the femtosecond laser phase mask method has weak birefringence characteristics, the wavelength difference of bragg reflection peaks corresponding to two orthogonal polarizations is very small, and it is difficult to realize a single-polarization fiber laser on a non-polarization-maintaining fiber by a wavelength selection matching method.
In summary, a new short straight-cavity single-polarization single-longitudinal-mode fiber laser and a method for manufacturing the same are needed.
Disclosure of Invention
The invention aims to provide a short straight cavity single polarization single longitudinal mode fiber laser and a preparation method thereof, so as to solve one or more technical problems. The preparation method can directly manufacture the short straight-cavity single-polarization single-longitudinal-mode fiber laser on the active fiber by using a femtosecond laser phase mask method.
In order to achieve the purpose, the invention adopts the following technical scheme:
the invention relates to a short straight cavity single polarization single longitudinal mode fiber laser, which comprises: the high-reflectivity optical fiber comprises a wavelength division multiplexer, a laser pumping source, an optical isolator and an active optical fiber, wherein a first fiber Bragg grating and a second fiber Bragg grating are engraved on the active optical fiber, the first fiber Bragg grating is used for forming a high-reflectivity cavity mirror, and the second fiber Bragg grating is used for forming an output cavity mirror; one end of the active optical fiber, on which a second fiber Bragg grating is engraved, is connected with a common end of the wavelength division multiplexer; the fast axis of the first fiber Bragg grating is parallel to the slow axis of the second fiber Bragg grating and corresponds to the same polarization state of light transmitted in the optical fiber; the slow axis of the first fiber Bragg grating is parallel to the fast axis of the second fiber Bragg grating and corresponds to another polarization state which is orthogonal to the polarization state; the wavelength of the Bragg reflection peak corresponding to the slow axis of the first fiber Bragg grating is equal to the wavelength of the Bragg reflection peak corresponding to the fast axis of the second fiber Bragg grating, so that the wavelength of the Bragg reflection peak corresponding to the fast axis of the first fiber Bragg grating is separated from the wavelength of the Bragg reflection peak corresponding to the slow axis of the second fiber Bragg grating.
The invention is further improved in that the pump end of the wavelength division multiplexer is connected with the laser pump source through a passive optical fiber, the laser end of the wavelength division multiplexer is connected with the optical isolator, and the common end of the wavelength division multiplexer is connected with one end of the active optical fiber, which is provided with a second fiber Bragg grating, through the passive optical fiber.
The invention has the further improvement that the reflectivity of the first fiber Bragg grating is more than or equal to 99 percent; the reflectivity of the second fiber Bragg grating is more than or equal to 95 percent.
A further improvement of the invention is that the active fiber is a non-polarization maintaining active fiber.
The invention discloses a preparation method of a short straight cavity single polarization single longitudinal mode fiber laser, which comprises the following steps:
wherein, the high-reflection cavity mirror, the output cavity mirror and the active optical fiber between the two cavity mirrors form a resonant cavity; the fast axis of the first fiber Bragg grating is parallel to the slow axis of the second fiber Bragg grating and corresponds to the same polarization mode A in the optical fiber; the slow axis of the first fiber Bragg grating is parallel to the fast axis of the second fiber Bragg grating and corresponds to the same polarization mode B in the optical fiber; polarization mode B is orthogonal to polarization mode a; the wavelength of the Bragg reflection peak corresponding to the slow axis of the first fiber Bragg grating is respectively equal to the wavelength of the Bragg reflection peak corresponding to the fast axis of the second fiber Bragg grating, so that the wavelength of the Bragg reflection peak corresponding to the fast axis of the first fiber Bragg grating is separated from the wavelength of the Bragg reflection peak corresponding to the slow axis of the second fiber Bragg grating.
The invention further improves the method and also comprises the following steps:
and 4, connecting the tail fiber at the end of the active fiber output cavity mirror with the common end of the wavelength division multiplexer, connecting the pumping end of the wavelength division multiplexer with a laser pumping source, and connecting the laser end of the wavelength division multiplexer with an optical isolator to form the fiber laser.
The invention has the further improvement that the exposure time of the first fiber Bragg grating is 20-50 s, and the reflectivity is more than or equal to 99 percent; the wavelength difference of Bragg reflection peaks corresponding to the fast axis and the slow axis of the first fiber Bragg grating is 0.06-0.08 nm;
the exposure time of the second fiber Bragg grating is 120-300 s, the reflectivity is not less than 95%, and the wavelength difference of Bragg reflection peaks corresponding to the fast axis and the slow axis is 0.2-0.3 nm.
The invention has the further improvement that the power of the femtosecond laser is 600 mW-1000 mW, the repetition frequency is 1kHz, and the wavelength is 800 nm; the femtosecond laser is linearly polarized light, and the polarization direction is parallel to the axis of the active optical fiber.
The invention discloses a preparation method of a short straight cavity single polarization single longitudinal mode fiber laser, which comprises the following steps:
in the step 1, removing a coating layer with the length of 1-2 cm at a preset position of an active optical fiber, and writing a first fiber Bragg grating on the active optical fiber core with the coating layer removed by using femtosecond laser to form a high-reflection cavity mirror of the optical fiber laser;
in the step 2, the active optical fiber is rotated by 90 degrees through a rotatable optical fiber clamp on the three-dimensional piezoelectric nano displacement platform, the resonance wavelength of the first fiber Bragg grating is shifted to a required preset wavelength through tension adjustment of the tension of the displacement platform, and then the first fiber Bragg grating is fixed;
the fast axis of the first fiber Bragg grating is parallel to the slow axis of the second fiber Bragg grating and corresponds to the same polarization state A of the transmitted light in the optical fiber; the slow axis of the first fiber Bragg grating is parallel to the fast axis of the second fiber Bragg grating and corresponds to the same polarization state B of the transmitted light in the optical fiber; polarization mode B is orthogonal to polarization mode a; the wavelength of a Bragg reflection peak corresponding to the slow axis of the first fiber Bragg grating is equal to the wavelength of a Bragg reflection peak corresponding to the fast axis of the second fiber Bragg grating; the wavelength of the Bragg reflection peak corresponding to the fast axis of the first fiber Bragg grating is separated from the wavelength of the Bragg reflection peak corresponding to the slow axis of the second fiber Bragg grating.
The exposure time of the first fiber Bragg grating is 20-50 s, and the reflectivity is more than or equal to 99%; the wavelength difference of Bragg reflection peaks corresponding to the fast axis and the slow axis of the first fiber Bragg grating is 0.06-0.08 nm; the exposure time of the second fiber Bragg grating is 120-300 s, the reflectivity is not less than 95%, and the wavelength difference of Bragg reflection peaks corresponding to the fast axis and the slow axis is 0.2-0.3 nm.
The further improvement of the invention is that in the step 2, the active optical fiber is rotated by 90 degrees through a rotatable optical fiber clamp on the three-dimensional piezoelectric nanometer displacement platform, and the resonance wavelength of the first fiber Bragg grating is shifted to the required preset wavelength through the tension adjustment of the tension of the displacement platform; the Bragg reflection peak wavelength of the stretched first fiber Bragg grating shifts 1.0-1.3 nm towards the long wave direction; in the step 3, the preset distance of the length of the translation cavity is 5mm-3cm, and the output of the single longitudinal mold is ensured; the overexposure writing comprises: and the overexposure time is 120-300 s, the reflection rate of the fiber Bragg grating engraved and written in the exposure reaches saturation, and then the reflection rate is reduced and gradually increased to reach the required preset reflection rate.
Compared with the prior art, the invention has the following beneficial effects:
according to the short straight cavity single-polarization single-longitudinal mode fiber laser, a first fiber Bragg grating and a second fiber Bragg grating are directly engraved on an active fiber, the fast axis of the first fiber Bragg grating is parallel to the slow axis of the second fiber Bragg grating, and the slow axis of the first fiber Bragg grating is parallel to the fast axis of the second fiber Bragg grating; the wavelength of the Bragg reflection peak corresponding to the slow axis of the first fiber Bragg grating is respectively equal to the wavelength of the Bragg reflection peak corresponding to the fast axis of the second fiber Bragg grating, and the wavelength of the Bragg reflection peak corresponding to the fast axis of the first fiber Bragg grating is separated from the wavelength of the Bragg reflection peak corresponding to the slow axis of the second fiber Bragg grating. Compared with the structure that the fast and slow axes of the first fiber Bragg grating and the second fiber Bragg grating are parallel to each other, the separation degree of the reflection peak of the polarized light orthogonal to the two resonant cavity mirrors in the resonant cavity is increased under the condition that the wavelengths of the reflection peaks of the polarized light parallel to the slow axis direction of the first fiber Bragg grating are the same, so that the short straight cavity fiber laser adopting the smaller double-refraction fiber Bragg grating can form single longitudinal mode output. In addition, the output cavity mirror prepared by the femtosecond laser overexposure method improves the birefringence characteristic of the fiber Bragg grating prepared by the phase mask plate method to the maximum extent. The invention is particularly suitable for the processing method with low birefringence of the prepared fiber Bragg grating by utilizing a femtosecond laser phase mask method.
According to the preparation method, the femtosecond laser is used for directly writing the fiber Bragg grating with the birefringence characteristic on the non-polarization-maintaining active fiber to form the laser resonant cavity; during writing, the directions of a fast axis and a slow axis of two fiber Bragg gratings serving as a high-reflectivity cavity mirror and a low-reflectivity cavity mirror are mutually orthogonal, a reflection peak corresponding to the slow axis of the high-reflectivity fiber Bragg grating is matched with a reflection peak corresponding to the fast axis of the second fiber Bragg grating by applying a tension force to change the period of the fiber Bragg grating, and the other two reflection peaks are relatively separated and increased. The slow axis of the first fiber Bragg grating and the fast axis of the second fiber Bragg grating correspond to the same polarization mode in the optical fiber, and the fast axis of the first fiber Bragg grating and the slow axis of the second fiber Bragg grating correspond to the same polarization mode in the optical fiber, so that the first fiber Bragg grating and the second fiber Bragg grating form resonance due to wavelength matching and generate laser; the latter two reflection peaks are separated by a large amount, and cannot form laser light. The invention realizes that the short-cavity single-polarization fiber laser is directly manufactured on the non-polarization-maintaining active fiber by using the femtosecond laser phase mask method, and compared with the existing method adopting the polarization-maintaining fiber and the polarization-maintaining fiber Bragg grating, the method has the advantages of lower cost and simpler processing method. In addition, the structure that the fast and slow optical axes of the first fiber Bragg grating and the second fiber Bragg grating are mutually orthogonal is adopted, and the interval between the wavelength and the Bragg reflection peak which is in the orthogonal polarization mode and does not form laser output is increased under the condition that the wavelength of the reflection peak of the resonant cavity in a certain polarization mode in the optical fiber is matched, so that the selectable cavity length is correspondingly increased, and the gain of the laser is increased.
The preparation method of the invention uses femtosecond laser to write a first fiber Bragg grating on an active fiber as a high-reflection cavity mirror; rotating the optical fiber by 90 degrees, and preparing a second fiber Bragg grating at the corresponding position of the active optical fiber in an overexposure mode to serve as an output cavity mirror; the fast and slow axes of the two Bragg gratings are mutually rotated by 90 degrees to form the optical fiber laserAn active cavity of an optical device. The fiber Bragg grating inscribed in the non-polarization-maintaining active fiber by utilizing the femtosecond laser has the birefringence characteristic; the wavelength of the Bragg reflection peak corresponding to the fast axis and the slow axis of the first fiber Bragg grating is lambda respectively1fAnd λ1s(λ1f<λ1s) The wavelengths of the Bragg reflection peaks corresponding to the fast axis and the slow axis of the second fiber Bragg grating are respectively lambda2fAnd λ2s(λ2f<λ2s) The processing method of overexposure can enhance the birefringence characteristics of the fiber Bragg grating, namely the reflection peaks lambda corresponding to two orthogonal polarization modes2fAnd λ2sA difference of (d); compared with the first fiber Bragg grating, the second fiber Bragg grating which is over-exposed and inscribed has the following advantages that the corresponding Bragg reflection peak wavelength of the fast and slow axes is larger: lambda [ alpha ]1f<λ2f，λ1s<λ2s. Before writing the second fiber Bragg grating, the fiber is stretched to make the slow axis Bragg reflection peak wavelength lambda of the first fiber Bragg grating1sAnd the slow axis Bragg reflection peak wavelength lambda of the output cavity mirror to be prepared2fMatching, and forming laser resonance because the slow axis of the first fiber Bragg grating and the fast axis of the second fiber Bragg grating correspond to the same polarization mode; the wavelength λ of the Bragg reflection peak of the two gratings corresponding to the other orthogonal polarization mode1fAnd λ2sThen far apart and no laser can be resonantly formed. Thereby creating a polarization output selection for the laser cavity. Due to the orthogonal arrangement of the fast and slow axes of the first fiber Bragg grating and the second fiber Bragg grating, the separation of the reflection peak pair in the orthogonal polarization state of the two cavity mirrors of the fiber laser is increased under the condition that the reflection peak pair in the polarization state of the fiber is matched, and the polarization selection is easier to realize compared with the condition that the fast and slow axes of the first fiber Bragg grating and the second fiber Bragg grating are consistent.
In the preparation method, the tail fiber at the output cavity mirror end of the active resonant cavity is connected with the common end of a wavelength division multiplexer, a pumping source of a laser is connected with the pumping end of the wavelength division multiplexer, and the laser end of the wavelength division multiplexer is connected with an optical isolator to form the complete structure of the optical fiber laser, thereby completing the preparation of the single longitudinal mode single polarization optical fiber laser.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art are briefly introduced below; it is obvious that the drawings in the following description are some embodiments of the invention, and that for a person skilled in the art, other drawings can be derived from them without inventive effort.
FIG. 1 is a schematic structural diagram of a short straight-cavity single-polarization single-longitudinal-mode fiber laser manufactured by the manufacturing method of the embodiment of the invention;
FIG. 2 is a schematic flow chart of a method for manufacturing a short straight-cavity single-polarization single-longitudinal-mode fiber laser according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of a processing apparatus for a method of fabricating a short straight cavity single polarization single longitudinal mode fiber laser according to an embodiment of the present invention;
FIG. 4 is a schematic diagram of the polarization direction of the transmission optical field and the directions of the fast axis and the slow axis of the fiber Bragg grating in a short straight-cavity single-polarization single-longitudinal-mode fiber laser according to an embodiment of the present invention;
fig. 5 is a schematic diagram of relative relationship between reflection spectra of polarization modes corresponding to fast and slow axes of a first fiber bragg grating and a second fiber bragg grating of a short straight cavity single polarization single longitudinal mode fiber laser according to an embodiment of the present invention;
FIG. 6 is a schematic diagram of the corresponding transmission spectra of p-light and s-light of the first fiber Bragg grating and the second fiber Bragg grating prepared in the embodiment of the present invention;
FIG. 7 is a schematic diagram of a single longitudinal mode, single polarization laser spectrum output by a fiber laser in accordance with an embodiment of the present invention;
in the figure, 1 is a laser pumping source; 2 is a passive optical fiber; 3 is an active optical fiber; 4 is a first fiber Bragg grating; 5 is a second fiber Bragg grating; 6 is a wavelength division multiplexer; 7 is an optical isolator; 8 is a rotatable optical fiber clamp; 9 is a three-dimensional piezoelectric nano displacement table; 10 is a three-dimensional macro-motion displacement platform; 11 is femtosecond laser; 12 is a cylindrical lens; 13 is a phase mask plate; 14 is a variable attenuator; 15 is a broadband light source point; 16 is a spectrum analyzer; 17 is the fiber grating for writing; 18 is an optical switch; 19 is a tension adjusting displacement table;
20 is the slow axis bragg reflection spectrum (p polarization) of the inscribed first fiber bragg grating; 21 is fast axis bragg reflection spectrum (s polarization) of the first fiber bragg grating; 22 is the slow axis bragg reflection spectrum (s polarization) of the inscribed second fiber bragg grating; 23 is the fast axis bragg reflection spectrum (p-polarization) of the inscribed second fiber bragg grating.
Detailed Description
In order to make the purpose, technical effect and technical solution of the embodiments of the present invention clearer, the following clearly and completely describes the technical solution of the embodiments of the present invention with reference to the drawings in the embodiments of the present invention; it is to be understood that the described embodiments are only some of the embodiments of the present invention. Other embodiments, which can be derived by one of ordinary skill in the art from the disclosed embodiments without inventive faculty, are intended to be within the scope of the invention.
Referring to fig. 1, fig. 1 is a schematic structural diagram of a fiber laser manufactured according to the present invention, which is composed of an active fiber 3, a first fiber bragg grating 4 and a second fiber bragg grating 5 on the active fiber 3, a wavelength division multiplexer 6, a laser pump source 1, an optical isolator 7, and a connection pigtail. The embodiment of the invention provides a short straight cavity single polarization single longitudinal mode fiber laser, which comprises: wavelength division multiplexing
The device comprises a user 6, a laser pumping source 1, an optical isolator 7 and an active optical fiber 3; a first fiber Bragg grating 4 and a second fiber Bragg grating 5 are engraved on the active fiber 3, the first fiber Bragg grating 4 is used for forming a high-reflection cavity mirror, and the second fiber Bragg grating 5 is used for forming an output cavity mirror; one end of the active optical fiber 3, on which the second fiber Bragg grating 5 is engraved, is connected with the common end of the wavelength division multiplexer 6; wherein, the fast axis of the first fiber Bragg grating 4 is parallel to the slow axis of the second fiber Bragg grating 5 and corresponds to the same polarization state A of the transmitted light in the optical fiber; the slow axis of the first fiber Bragg grating 4 is parallel to the fast axis of the second fiber Bragg grating 5 and corresponds to the same polarization state B of the transmitted light in the optical fiber; polarization mode B is orthogonal to polarization mode a; the wavelength of the Bragg reflection peak corresponding to the slow axis of the first fiber Bragg grating 4 is equal to the wavelength of the Bragg reflection peak corresponding to the fast axis of the second fiber Bragg grating 5, and the wavelength of the Bragg reflection peak corresponding to the fast axis of the first fiber Bragg grating 4 is separated from the wavelength of the Bragg reflection peak corresponding to the slow axis of the second fiber Bragg grating 5.
Preferably, the active fiber 3 is a non-polarization maintaining active fiber 3, including a non-polarization maintaining erbium doped fiber, a ytterbium doped fiber, a erbium ytterbium co-doped fiber and a thulium doped fiber. The laser pumping source 1 is a semiconductor laser pumping source.
Preferably, the exposure time of the first fiber Bragg grating 4 is 20s-50s, the reflectivity is more than or equal to 99%, the birefringence is smaller than that of the second fiber Bragg grating 5, and the wavelength difference of Bragg reflection peaks corresponding to the fast axis and the slow axis is 0.06 nm-0.08 nm. The second fiber Bragg grating 5 serving as the output cavity mirror is prepared by an overexposure processing method, the exposure time is 120-300 s, the second fiber Bragg grating has high birefringence, the reflectivity is not less than 95%, and the wavelength difference of Bragg reflection peaks corresponding to a fast axis and a slow axis is 0.2-0.3 nm.
The preparation method of the short straight cavity single polarization single longitudinal mode fiber laser comprises the following steps: the method comprises the steps of directly writing a first fiber Bragg grating 4 on a fiber core of an active fiber 3 by using a femtosecond laser 11 to form a high-reflection cavity mirror of a fiber laser, rotating the fiber by 90 degrees, stretching the fiber to ensure that the wavelength of a Bragg reflection peak of the first fiber Bragg grating 4 is shifted to a long wave direction for determining a certain amount and then fixing, moving the fiber to ensure that the writing femtosecond laser 11 is aligned with the corresponding position of an output cavity mirror of the fiber laser, writing a second fiber Bragg grating 5 by using the excessive exposure of the femtosecond laser 11, connecting a tail fiber at the output mirror end of the active fiber 3 with a common end of a wavelength division multiplexer 6, connecting a pumping end of the wavelength division multiplexer 6 with a single-mode laser pumping source 1, and connecting a laser end of the wavelength division multiplexer 6 with an.
Preferably, the fiber bragg gratings as the high-reflectivity cavity mirror and the output cavity mirror of the fiber laser are directly prepared in the active fiber 3 by using the femtosecond laser 11, and have a birefringence characteristic that each grating has two bragg reflection peaks corresponding to two polarization states orthogonal to each other, and the directions of the fast axis and the slow axis of the first fiber bragg grating 4 are orthogonal to the directions of the fast axis and the slow axis of the second fiber bragg grating 5 as the output cavity mirror.
The embodiment of the invention discloses a preparation method of a short straight cavity single polarization single longitudinal mode fiber laser, which comprises the steps of utilizing a femtosecond laser phase mask method to write a first fiber Bragg grating on a non-polarization-maintaining active fiber to serve as a high-reflection cavity mirror, rotating the fiber by 90 degrees, stretching the fiber, and then utilizing femtosecond laser to directly write a second fiber Bragg grating with low reflectivity on the active fiber through overexposure, wherein the fast and slow axis directions of the high-reflectivity Bragg gratings are mutually orthogonal, and the slow axis Bragg reflection peak wavelength of the first fiber Bragg grating is consistent with the fast axis Bragg reflection peak wavelength of the low-reflection second fiber Bragg grating. And connecting the active optical fiber pigtail at the output end with the common end of the wavelength division multiplexer, connecting a semiconductor laser pumping source with the pumping end of the wavelength division multiplexer, and connecting the signal end of the wavelength division multiplexer with an optical isolator to form the single-longitudinal-mode single-polarization short straight-cavity optical fiber laser. The invention prepares the short straight cavity fiber laser with polarization output characteristic in the non-polarization active fiber, solves the problem that the space of the reflection peak of the fast and slow axes of the Bragg grating is too small to carry out polarization selection by utilizing the femtosecond laser phase mask method to write in the non-polarization-maintaining fiber, and has the advantages of low price, simple processing method, no welding, no optical fiber material selectivity, high temperature resistance and the like.
The preparation method of the short straight cavity single polarization single longitudinal mode fiber laser comprises the following steps:
1) removing a coating layer of the active optical fiber 3 with the length of 1 cm-2 cm, and fixing the coating layer on a three-dimensional piezoelectric nano displacement table 9;
2) the first fiber Bragg grating 4 is engraved on the part of the active fiber 3 where the coating is removed by using femtosecond laser 11;
3) rotating the active optical fiber 3 by 90 degrees, stretching to shift the resonance wavelength of the first fiber Bragg grating 4 to the required wavelength, and then fixing the optical fiber;
4) translating the optical fiber by a distance corresponding to the cavity length by using a three-dimensional macro-motion displacement platform 10, and aligning the focus of the femtosecond laser 11 to the position of an output cavity mirror to be etched;
5) the femtosecond laser 11 is used for writing the fiber Bragg grating on the active fiber 3 through overexposure until the reflectivity of the fiber Bragg grating reaches a required value, and the writing of the low-reflectivity output fiber Bragg grating is completed;
6) and (3) fusing the tail fiber of the active fiber 3 at the end of the second fiber Bragg grating 5 with the common end of the wavelength division multiplexer 6, connecting the pumping end of the wavelength division multiplexer 6 with the pumping source tail fiber of the laser, and connecting the laser end of the wavelength division multiplexer 6 with the optical isolator 7 to form the fiber laser.
The steps of the specific embodiment include:
in the step 1), the coating layer can not be removed, and then the femtosecond laser 11 is used for etching the grating on the fiber core of the optical fiber at intervals of the coating layer.
In the step 1), one end of the active optical fiber 3 is generally connected with a broadband light source, the other end of the active optical fiber is connected with a spectrum analyzer 16, and the spectrum analyzer 16 is used for measuring the transmission spectrum to monitor the writing process of the fiber Bragg grating.
The exposure time of the first fiber Bragg grating 4 in the step 2) is 20s-50s, the reflectivity is more than or equal to 99%, the birefringence is smaller than that of the second fiber Bragg grating 5, and the wavelength difference of Bragg reflection peaks corresponding to a fast axis and a slow axis is 0.06 nm-0.08 nm; i.e. the inscribed first fibre bragg grating 4 has a weak birefringence characteristic.
After the active optical fiber 3 is rotated by 90 degrees in the step 3), the stretching aims to match the Bragg reflection peak corresponding to the slow axis of the first fiber Bragg grating 4 with the Bragg reflection peak corresponding to the fast axis of the second fiber Bragg grating 5, so that the fiber Bragg reflection peaks corresponding to the fast axis of the first fiber Bragg grating 4 and the slow axis of the high-birefringence second fiber Bragg grating 5 are separated and increased, and the wavelength of the Bragg reflection peak of the stretched first fiber Bragg grating 4 shifts 1.0-1.3 nm towards the long wave direction. Specifically, the stretching is performed under the monitoring of a fiber bragg grating transmission spectrum measuring system, and the stretching standard of the first fiber bragg grating 4 is that a bragg reflection peak corresponding to a slow axis of the stretched first fiber bragg grating corresponds to a bragg reflection peak corresponding to a fast axis of the inscribed high-birefringence second fiber bragg grating.
The corresponding distance of the fiber translation cavity length in the step 4) is 5mm-3cm, so that single longitudinal mode output is ensured.
The second fiber Bragg grating 5 serving as the output cavity mirror in the step 5) has high birefringence, the reflectivity is more than or equal to 95%, and the wavelength difference of Bragg reflection peaks corresponding to the fast axis and the slow axis is 0.2-0.3 nm.
The overexposure time in the step 5) is 120-300 s, the reflection rate of the fiber Bragg grating which is inscribed in the exposure reaches saturation firstly, then is reduced and gradually rises to reach the required reflectivity, and the Bragg wavelength is redshifted compared with the saturated state after overexposure. The second fiber bragg grating 5 is prepared by overexposure in order to improve its birefringence.
In the embodiment of the invention, the power of the femtosecond laser 11 used for writing is 600 mW-1000 mW, the repetition frequency is 1kHz, the wavelength is 800nm, the femtosecond laser 11 is linearly polarized light, and the polarization direction is parallel to the optical fiber axis.
In the embodiment of the invention, the resonance wavelength of the short-cavity single-longitudinal-mode single-polarization fiber laser is determined by the Bragg reflection peaks of the slow axis of the first fiber Bragg grating 4 and the fast axis of the second fiber Bragg grating 5, and the Bragg reflection peaks have the same wavelength and correspond to the same polarization mode. The orthogonal polarization mode of the optical fiber is corresponding to the fast axis of the first fiber Bragg grating 4 and the slow axis of the high-birefringence second fiber Bragg grating 5, the wavelength difference of Bragg reflection peaks corresponding to the two is increased, the separation of the reflection peaks is increased, and laser cannot be formed. In the embodiment of the invention, the wavelength separation amount of the Bragg reflection peak of two Bragg gratings corresponding to another orthogonal polarization mode is increased by a method of enabling the directions of the slow axis of the first fiber Bragg grating 4 and the fast axis of the second fiber Bragg grating 5 to be consistent with the wavelength of the Bragg reflection peak of the corresponding polarization mode, and the distance between the two fiber Bragg gratings, namely the cavity length of a fiber laser resonant cavity, is controlled, so that the fiber laser output of a single longitudinal mode and single polarization is realized in a non-polarization-maintaining fiber. The femtosecond laser 11 phase mask method is adopted to write the fiber Bragg grating on the active fiber 3, thereby avoiding welding and being applicable to various different active fibers 3, the method does not need complex equipment such as expensive double refraction welding machines, can realize single longitudinal mode and single polarization fiber laser on the common non-polarization-maintaining fiber with low price, and has the advantages of simple operation, no welding of the cavity mirror, low price and good stability.
Referring to fig. 2, a method for manufacturing a short straight cavity single polarization single longitudinal mode fiber laser according to an embodiment of the present invention is a method for directly manufacturing a short cavity single polarization single longitudinal mode fiber laser on a non-polarization maintaining active fiber 3, and includes:
welding two ends of a section of active optical fiber 3 with the passive optical fiber 2 for transmission, and stripping a section of the coating layer of the active optical fiber 3;
under the detection of a spectrum analyzer 16, a femtosecond laser 11 is used for writing a first fiber Bragg grating 4 at one end of an active fiber 3, then the fiber is rotated by 90 degrees, and the wavelength position of a slow axis reflection peak of the written first fiber Bragg grating 4 is shifted to be corresponding to (equal to) the wavelength of a fast axis reflection peak of a second fiber Bragg grating 5 to be prepared by applying a pulling force;
writing a low-reflectivity high-birefringence fiber Bragg grating at the position of an output cavity mirror by using femtosecond laser 11 in an overexposure mode, thereby preparing a short-cavity fiber laser resonant cavity;
the active optical fiber 3 tail fiber of the output cavity mirror end is connected with the common end of the wavelength division multiplexer 6, the pumping end of the wavelength division multiplexer 6 is connected with the laser pumping source 1, and the laser end of the wavelength division multiplexer 6 is connected with the optical isolator 7 to form the short-cavity optical fiber laser.
Referring to fig. 2 and fig. 3, a method for manufacturing a short straight cavity single polarization single longitudinal mode fiber laser according to an embodiment of the present invention is implemented by the apparatus shown in fig. 3, and specifically includes the following steps:
1) connecting one end of a section of active optical fiber 3 with a tail fiber output by a broadband light source point 15, welding one end of the active optical fiber with a jumper wire connected with a spectrum analyzer 16, stripping a coating layer of the active optical fiber 3 by 1-2 cm by using a wire stripper, fixing the active optical fiber 3 on a three-dimensional piezoelectric nano micro-displacement table by using a rotatable optical fiber clamp 8, and fixing the three-dimensional piezoelectric nano-micro-displacement table on a three-dimensional macro-motion displacement table 10;
2) the position of a high-reflectivity cavity mirror to be etched of the active optical fiber 3 is aligned to the femtosecond laser 11 by using the three-dimensional macro-motion displacement platform 10, the power of the femtosecond laser 11, the scanning speed and the exposure time of the three-dimensional piezoelectric nano micro-displacement platform are set, and the saturated first fiber Bragg grating 4 is etched on the active optical fiber 3 by using the femtosecond laser 11.
3) The active optical fiber 3 is rotated by 90 degrees, and the active optical fiber 3 is stretched by the tension device, so that the Bragg reflection peak wavelength corresponding to the slow axis of the active optical fiber 3 deviates a certain amount to the long wave direction, and is consistent with the fast axis Bragg reflection peak wavelength of the low-reflectivity high-birefringence second optical fiber Bragg grating 5 to be inscribed. Wherein, the tension device can adopt a tension adjusting displacement table 19.
4) The second fiber Bragg grating 5 is prepared on the fiber Bragg grating by an overexposure method by using a femtosecond laser 11 phase mask method, the fiber Bragg grating has higher birefringence, and the reflectivity of the inscribed fiber Bragg grating is detected by using a broadband light source and a spectrum analyzer 16.
5) The connection point of the prepared fiber laser resonant cavity and the broadband light source and the spectrum analyzer 16 is disconnected, the tail fiber at the end of the second fiber Bragg grating 5 is connected with the common end of the wavelength division multiplexer 6, the pumping end of the wavelength division multiplexer 6 is connected with the pumping source output tail fiber of the laser, and the signal end of the wavelength division multiplexer 6 is connected with the optical isolator 7, so that the short-cavity fiber laser system is formed.
Referring to fig. 3, the processing device for implementing the preparation method of the present invention is composed of a rotatable optical fiber clamp 8, a three-dimensional piezoelectric nano displacement stage 9, a three-dimensional macro-motion displacement stage 10, a femtosecond laser 11, a cylindrical lens 12, a phase mask 13, a variable attenuator 14, a broadband light source, a spectrum analyzer 16, an optical switch 18, and a tension adjusting device. The apparatus of figure 3 is used to produce a written fibre grating 17 in the active fibre 3.
Referring to fig. 4 and 5, fig. 4 shows the fast and slow axes of the propagating optical electric field and fiber bragg grating in a short cavity fiber laser.
Referring to fig. 1 to 7, an embodiment of the present invention takes a short-cavity single-longitudinal-mode single-polarization fiber laser fabricated in an ytterbium-doped fiber by using a femtosecond laser 11 as an example, as shown in fig. 3, specifically as follows:
raw materials: the erbium-doped fiber Liekki Er80-8/125, a 980nm/1550nm wavelength division multiplexer and a 976nm semiconductor laser single-mode pumping source.
The preparation steps of the short straight cavity single longitudinal mode single polarization fiber laser provided by the embodiment of the invention are elaborated as follows:
(1) one end of a section of active optical fiber 3 is welded with a broadband light source output optical fiber, and the other end is welded with a 16-type jumper of a spectrum analyzer, the coating of the active optical fiber 3 is removed by 1 cm-2 cm, and then the active optical fiber is fixed on a three-dimensional piezoelectric nano displacement table 9 by a rotatable optical fiber clamp 8;
(2) the method comprises the steps of aligning the position of a high-reflectivity cavity mirror to be etched of an active optical fiber 3 to the focus of a femtosecond laser 11 by using a three-dimensional macro-motion displacement platform 10, setting the power of the femtosecond laser 11 to be 600mW, scanning the active optical fiber 3 along the x axis within the range of 20 microns, scanning the active optical fiber within the period of 9s, and etching the active optical fiber 3 for 30s to form a saturated first fiber Bragg grating 4, wherein the etching state of the fiber Bragg grating is monitored by using an ASE broadband light source and a spectrum analyzer 16. The transmission spectra of the p-light and the s-light of the first fiber bragg grating 4 are shown in fig. 6, and correspond to the fast-axis bragg reflection peak and the slow-axis bragg reflection peak, respectively.
(3) The active optical fiber 3 is rotated by 90 degrees by using the rotatable optical fiber clamp 8, the active optical fiber 3 is stretched by the tension adjusting device under the monitoring of the spectrum analyzer 16, so that the wavelength of the reflection peak corresponding to the slow axis of the first fiber Bragg grating 4 is deviated to the long wave direction, and is consistent with the wavelength of the reflection peak of the fast axis of the second fiber Bragg grating 5 to be written.
(4) The active optical fiber 3 is translated by 5mm along a z axis by using the three-dimensional macro-motion displacement platform 10, the position of a cavity mirror with low reflectivity to be etched is aligned to the focus of the femtosecond laser 11, the power of the femtosecond laser 11 is set to be 600mW, the scanning range is 20 microns along an x axis, the scanning period is 9s, the exposure time is 220s, and the second fiber Bragg grating 5 is prepared on the fiber Bragg grating by using the femtosecond laser 11 phase mask method through an overexposure method, the fiber Bragg grating has higher birefringence, and the saturated first fiber Bragg grating 4 is etched on the active optical fiber 3. The writing process of the fiber bragg grating is monitored by means of an ASE light source and a spectrum analyzer 16. The transmission spectra of p-light and s-light of the second fiber bragg grating 5 are shown in fig. 6, and correspond to the slow-axis and fast-axis bragg reflection peaks thereof, respectively.
(5) The tail fiber of the low-reflectivity end of the active fiber 3 with the well-engraved fiber Bragg grating is welded with the tail fiber of the common end of the wavelength division multiplexer 6, the pumping end of the wavelength division multiplexer 6 is welded with the output tail fiber of the semiconductor laser pumping source 1, and the signal end of the wavelength division multiplexer 6 is connected with the optical isolator 7 to form the short straight cavity single longitudinal mode single polarization fiber laser. The output spectrum of the laser is shown in FIG. 7
In the embodiment of the present invention, the second fiber bragg grating 5 has high birefringence through an overexposure processing method, so that the wavelength interval between two reflection peaks is increased. Particularly, the fast axis and the slow axis of the first fiber bragg grating 4 and the second fiber bragg grating 5 are mutually orthogonal, and the slow axis reflection peak of the first fiber bragg grating 4 corresponds to the fast axis reflection peak of the second fiber bragg grating 5, so that resonance of a polarization mode consistent with the polarization direction is formed, and laser is output; the separation between the fast axis reflection peak of the first fiber bragg grating 4 and the slow axis reflection peak of the second fiber bragg grating 5 increases, and laser light cannot be generated, thereby forming single polarization output.
In summary, the present invention is directed to a method for directly fabricating a short straight-cavity single-polarization single-longitudinal mode fiber laser on a non-polarization-maintaining active fiber by using a femtosecond laser phase mask method, so as to overcome the problems in the prior art. The preparation method comprises the steps of utilizing femtosecond laser to write a first fiber Bragg grating on a common active fiber as a high-reflection cavity mirror, rotating the fiber by 90 degrees, preparing a second fiber Bragg grating at the corresponding position of the active fiber in an overexposure mode as an output cavity mirror, and rotating the fast axis and the slow axis of the two fiber Bragg gratings by 90 degrees to form an active resonant cavity of the fiber laser. The fiber Bragg grating inscribed in the non-polarization-maintaining active fiber by femtosecond laser has double refraction characteristics, and the wavelengths of Bragg reflection peaks corresponding to the fast axis and the slow axis of the first fiber Bragg grating are respectively lambda1fAnd λ1s(λ1f<λ1s) The fast axis and the slow axis of the second fiber Bragg grating correspond to each otherRespectively, the wavelength of the Bragg reflection peak is lambda2fAnd λ2s(λ2f<λ2s). The processing method of overexposure enhances the birefringence characteristics of the fiber Bragg grating, namely the reflection peaks lambda corresponding to two orthogonal polarization modes2fAnd λ2sThe difference of (a). Compared with the first fiber Bragg grating, the second fiber Bragg grating which is over-exposed and inscribed has the following advantages that the Bragg reflection peak wavelength corresponding to the fast axis and the slow axis is larger: lambda [ alpha ]1f<λ2f，λ1s<λ2s. Before writing the second fiber Bragg grating, the fiber is stretched to make the first fiber Bragg grating have the slow axis Bragg reflection peak wavelength lambda1sAnd the slow axis Bragg reflection peak wavelength lambda of the output cavity mirror to be prepared2fCorrespondingly, the slow axis of the first fiber Bragg grating and the fast axis of the second fiber Bragg grating correspond to the same polarization mode, so that laser resonance is formed; the wavelength λ of the Bragg reflection peak of the two gratings corresponding to the other orthogonal polarization mode1fAnd λ2sIt is far apart and laser resonance cannot be formed. Thereby creating a polarization output selection for the laser cavity. The tail fiber at the output cavity mirror end of the active resonant cavity is connected with the common end of a wavelength division multiplexer, a pumping source of a laser is connected with the pumping end of the wavelength division multiplexer, the laser end of the wavelength division multiplexer is connected with an optical isolator to form a complete structure of the optical fiber laser, and finally the single longitudinal mode single polarization optical fiber laser is output. The invention increases the wavelength separation of the Bragg reflection peak of two Bragg gratings corresponding to another orthogonal polarization mode by a method of making the directions of the slow axis of the first fiber Bragg grating and the fast axis of the second fiber Bragg grating and the wavelength of the corresponding Bragg reflection peak consistent, controls the distance between the two fiber Bragg gratings and realizes the output of single longitudinal mode and single polarization fiber laser in a non-polarization-maintaining fiber. The femtosecond laser phase mask method is adopted to write the fiber Bragg grating on the active fiber, thereby avoiding welding and being applicable to various different active fibers. The inventionThe method comprises directly writing fiber Bragg grating with birefringence on non-polarization-maintaining active fiber by femtosecond laser to form laser resonant cavity, making the fast axis and slow axis directions of two fiber Bragg gratings as high-reflection and low-reflection cavity mirror orthogonal during writing, and matching the reflection peak corresponding to the slow axis of the high-reflectivity fiber Bragg grating with the reflection peak corresponding to the fast axis of the second fiber Bragg grating by changing the period of the fiber Bragg grating by applying tension to form resonance, wherein the other two reflection peaks are separated and enlarged and cannot form laser. And the active resonant cavity is connected with a pumping source through a wavelength division multiplexer to form a laser, and finally, the single longitudinal mode and single polarization fiber laser is realized. The invention realizes the direct manufacture of the short-cavity single-polarization fiber laser on the non-polarization-maintaining active fiber, and has lower cost and simpler processing method compared with the existing method adopting the polarization-maintaining fiber and the polarization-maintaining fiber Bragg grating. In addition, the structure that the fast and slow optical axes of the first fiber Bragg grating and the second fiber Bragg grating are mutually orthogonal is adopted, and the wavelength interval of the Bragg reflection peak of the laser is not increased, so that the selectable cavity length is correspondingly increased, and the gain of the laser is increased.
Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can make modifications and equivalents to the embodiments of the present invention without departing from the spirit and scope of the present invention, which is set forth in the claims of the present application.
Claims (10)
1. A method for preparing a short straight cavity single polarization single longitudinal mode fiber laser is characterized by comprising the following steps:
step 1, a femtosecond laser (11) is used for writing a first fiber Bragg grating (4) on a fiber core of an active fiber (3) to form a high-reflection cavity mirror of the fiber laser;
step 2, rotating the active optical fiber (3) processed in the step 1 by 90 degrees, stretching the active optical fiber (3) to enable the Bragg reflection peak wavelength of the first fiber Bragg grating (4) to shift to the long wave direction by a preset amount and then fixing;
step 3, aligning the femtosecond laser (11) for writing to the preset output cavity mirror position of the fiber laser, and writing a second fiber Bragg grating (5) by using the femtosecond laser (11) to perform overexposure lithography to form an output cavity mirror; the output cavity mirror end of the active optical fiber (3) is used for being connected with the common end of the wavelength division multiplexer (6) to form an optical fiber laser;
wherein, the high-reflection cavity mirror, the output cavity mirror and the active optical fiber (3) between the two cavity mirrors form a resonant cavity; the fast axis of the first fiber Bragg grating (4) is parallel to the slow axis of the second fiber Bragg grating (5) and corresponds to the same polarization mode A in the optical fiber; the slow axis of the first fiber Bragg grating (4) is parallel to the fast axis of the second fiber Bragg grating (5) and corresponds to the same polarization mode B in the optical fiber; polarization mode B is orthogonal to polarization mode a; the Bragg reflection peak wavelength corresponding to the slow axis of the first fiber Bragg grating (4) is equal to the Bragg reflection peak wavelength corresponding to the fast axis of the second fiber Bragg grating (5), and the Bragg reflection peak wavelength corresponding to the fast axis of the first fiber Bragg grating (4) is separated from the Bragg reflection peak wavelength corresponding to the slow axis of the second fiber Bragg grating (5).
2. The method for preparing a short straight cavity single polarization single longitudinal mode fiber laser according to claim 1, further comprising:
and 4, connecting the tail fiber of the output cavity mirror end of the active fiber (3) with the common end of a wavelength division multiplexer (6), connecting the pumping end of the wavelength division multiplexer (6) with a laser pumping source (1), and connecting the laser end of the wavelength division multiplexer (6) with an optical isolator (7) to form the fiber laser.
3. The method for preparing the short straight cavity single polarization single longitudinal mode fiber laser device according to claim 1, wherein the exposure time of the first fiber Bragg grating (4) is 20-50 s, and the reflectivity is more than or equal to 99%; the wavelength difference of Bragg reflection peaks corresponding to the fast axis and the slow axis of the first fiber Bragg grating (4) is 0.06-0.08 nm;
the exposure time of the second fiber Bragg grating (5) is 120-300 s, the reflectivity is not less than 95%, and the wavelength difference of Bragg reflection peaks corresponding to the fast axis and the slow axis is 0.2-0.3 nm.
4. The method for preparing the short straight cavity single polarization single longitudinal mode optical fiber laser device according to claim 1, wherein the power of the femtosecond laser (11) is 600 mW-1000 mW, the repetition frequency is 1kHz, and the wavelength is 800 nm; the femtosecond laser (11) is linearly polarized light, and the polarization direction is parallel to the axis of the active optical fiber (3).
5. A short straight cavity single polarization single longitudinal mode fiber laser, characterized in that, it is prepared by the method of claim 1, the short straight cavity single polarization single longitudinal mode fiber laser comprises: the high-reflectivity optical fiber is characterized in that a first fiber Bragg grating (4) and a second fiber Bragg grating (5) are inscribed on the active optical fiber (3), the first fiber Bragg grating (4) is used for forming a high-reflectivity cavity mirror, and the second fiber Bragg grating (5) is used for forming an output cavity mirror; one end of the active optical fiber (3) on which a second fiber Bragg grating (5) is engraved is connected with the common end of the wavelength division multiplexer (6);
the fast axis of the first fiber Bragg grating (4) is parallel to the slow axis of the second fiber Bragg grating (5) and corresponds to the same polarization mode A in the optical fiber; the slow axis of the first fiber Bragg grating (4) is parallel to the fast axis of the second fiber Bragg grating (5) and corresponds to the same polarization mode B in the optical fiber; polarization mode B is orthogonal to polarization mode a;
the Bragg reflection peak wavelength corresponding to the slow axis of the first fiber Bragg grating (4) is equal to the Bragg reflection peak wavelength corresponding to the fast axis of the second fiber Bragg grating (5); the Bragg reflection peak wavelength corresponding to the fast axis of the first fiber Bragg grating (4) is separated from the Bragg reflection peak wavelength corresponding to the slow axis of the second fiber Bragg grating (5).
6. A short straight cavity single polarization single longitudinal mode fiber laser according to claim 5, characterized in that the pump end of the wavelength division multiplexer (6) is connected with the laser pump source (1) through a passive fiber (2), the laser end of the wavelength division multiplexer (6) is connected with the optical isolator (7), and the common end of the wavelength division multiplexer (6) is connected with the end of the active fiber (3) engraved with the second fiber Bragg grating (5) through the passive fiber (2).
7. A short straight cavity single polarization single longitudinal mode fiber laser according to claim 5, characterized in that the reflectivity of the first fiber Bragg grating (4) is equal to or more than 99%; the reflectivity of the second fiber Bragg grating (5) is more than or equal to 95 percent.
8. A short straight cavity single polarization single longitudinal mode fiber laser according to claim 5, characterized in that the active fiber (3) is a non-polarization maintaining active fiber.
9. A method for preparing a short straight cavity single polarization single longitudinal mode fiber laser is characterized by comprising the following steps:
in the step 1, removing a coating layer with the length of 1-2 cm at a preset position of an active optical fiber (3), and writing a first fiber Bragg grating (4) on the fiber core of the active optical fiber (3) with the coating layer removed by using a femtosecond laser (11) to form a high-reflection cavity mirror of the optical fiber laser;
in the step 2, the active optical fiber (3) is rotated by 90 degrees through a rotatable optical fiber clamp (8) on a three-dimensional piezoelectric nano displacement platform (9), and the first fiber Bragg grating (4) resonance wavelength is shifted to a required preset wavelength through the stretching of a tension adjusting displacement platform (19) and then is fixed;
in the step 3, the active optical fiber (3) is translated by a preset cavity length distance through the three-dimensional macro-motion displacement platform (10), so that the focus of the femtosecond laser (11) is aligned to the position of an output cavity mirror to be etched; the femtosecond laser (11) is used for writing the second fiber Bragg grating (5) on the active fiber (3) through overexposure until the reflectivity of the second fiber Bragg grating (5) reaches a required value, so that the writing of the low-reflectivity output fiber Bragg grating is completed;
step 4, welding the tail fiber of the active fiber (3) at the end of the second fiber Bragg grating (5) with the common end of a wavelength division multiplexer (6), connecting the pump end of the wavelength division multiplexer (6) with the tail fiber of a laser pump source (1), and connecting the laser end of the wavelength division multiplexer (6) with an optical isolator (7) to form a fiber laser;
wherein the fast axis of the first fiber Bragg grating (4) is parallel to the slow axis of the second fiber Bragg grating (5) and corresponds to the same polarization state A of the light transmitted in the optical fiber, and the slow axis of the first fiber Bragg grating (4) is parallel to the fast axis of the second fiber Bragg grating (5) and corresponds to the same polarization state B of the light transmitted in the optical fiber; polarization mode B is orthogonal to polarization mode a; the wavelength of a Bragg reflection peak corresponding to the slow axis of the first fiber Bragg grating (4) is respectively equal to the wavelength of a Bragg reflection peak corresponding to the fast axis of the second fiber Bragg grating (5), and the wavelength of the Bragg reflection peak corresponding to the fast axis of the first fiber Bragg grating (4) is separated from the wavelength of the Bragg reflection peak corresponding to the slow axis of the second fiber Bragg grating (5);
the exposure time of the first fiber Bragg grating (4) is 20-50 s, and the reflectivity is more than or equal to 99%; the wavelength difference of Bragg reflection peaks corresponding to the fast axis and the slow axis of the first fiber Bragg grating (4) is 0.06-0.08 nm; the exposure time of the second fiber Bragg grating (5) is 120-300 s, the reflectivity is not less than 95%, and the wavelength difference of Bragg reflection peaks corresponding to the fast axis and the slow axis is 0.2-0.3 nm.
10. The method for manufacturing a short straight cavity single polarization single longitudinal mode fiber laser according to claim 9,
in the step 2, the first fiber Bragg grating (4) is shifted to the required preset wavelength by stretching the tension adjusting displacement table (19); the Bragg reflection peak wavelength of the stretched first fiber Bragg grating (4) deviates 1.0-1.3 nm towards the long wave direction;
in the step 3, the preset distance of the length of the translation cavity is 5mm-3cm, and the output of the single longitudinal mold is ensured;
the overexposure writing comprises: and the overexposure time is 120-300 s, the reflection rate of the fiber Bragg grating engraved in the exposure reaches saturation, then the reflection rate is reduced and gradually increased to reach the required preset reflection rate, and the fiber Bragg wavelength is in red shift compared with the fiber Bragg wavelength in a saturated state.
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