Ultra-stable repetition rate all-fiber nonlinear optical spectrum analyzer fast-tunable ultrashort pulse seed source

Abstract

The application discloses a kind of ultra-stable heavy frequency all-fiber nonlinear spectrometer fast tuning ultra-short pulse seed source, including full polarization maintaining linear cavity, the full polarization maintaining linear cavity further includes pump laser diode (1), wavelength division multiplexer (2), polarization maintaining gain optical fiber (3), polarization maintaining fiber Fabry-Perot electric tuning filter (4), reflective semiconductor saturable absorber mirror (5) and polarization maintaining fiber linear chirped grating (6), constitute full polarization maintaining linear cavity;Wavelength-independent linear laser cavity length is realized by polarization maintaining fiber linear chirped grating, and fast electric tuning is realized by polarization maintaining fiber Fabry-Perot electric tuning filter.The application solves the problem that the output pulse frequency of ultra-short pulse seed source cannot be stably maintained when fast wide wavelength tuning range.

Description

Technical Field

[0001] This invention relates to the field of ultrashort pulse laser technology, specifically to a fast-tuned ultrashort pulse seed source for an all-fiber nonlinear spectrometer. Background Technology

[0002] Tunable ultrashort pulse fiber lasers have wide applications in nonlinear biomedical imaging, such as coherent Raman spectroscopy, selective excitation of molecular vibrational spectra, two-photon absorption for excitation of different fluorescent molecules, and second harmonic detection of different molecular structures. However, when a wide wavelength tuning range is required for a tunable ultrashort pulse fiber laser, the fixed fiber length and the dispersion parameters of the fiber material cause variations in the cavity length seen at different wavelengths. This results in different round-trip times within the cavity for different wavelengths, leading to dynamic changes in the repetition rate (RF) of the output laser pulse. The inability to maintain a constant RF due to wavelength tuning significantly limits the use of ultrashort pulse fiber lasers as synchronous pump sources and for high-precision synchronization with lasers of other wavelengths. While using an intracavity mechanical structure to dynamically adjust the delay line according to wavelength can achieve stable RF output, this method is cumbersome, has high losses, and the spatial optical path structure results in poor stability, limiting the tuning speed to the millisecond level. Summary of the Invention

[0003] To overcome the shortcomings of existing technologies, this invention aims to propose a fast-tuning ultrashort pulse seed source for an all-fiber nonlinear spectrometer with ultra-stable repetition rate. It adopts a polarization-maintaining linear cavity structure design to realize an ultrashort pulse seed source with a fast and wide wavelength tuning range, and outputs a stable pulse repetition rate during the fast and wide wavelength tuning process.

[0004] To achieve the above-mentioned objectives, the present invention proposes the following technical solution:

[0005] In a first aspect, the present invention provides a fast-tuned ultrashort pulse seed source for an ultra-stable repetition rate all-fiber nonlinear spectrometer, characterized in that it comprises a fully polarization-maintaining linear cavity, wherein the fully polarization-maintaining linear cavity further comprises a 980 nm pump laser diode 1, a wavelength division multiplexer 2 and its reflection end 21, common end 22, transmission end 23, polarization-maintaining gain fiber 3, a polarization-maintaining fiber Fabry-Perot electrically tuned filter 4, a reflective semiconductor saturable absorber mirror 5, and a polarization-maintaining fiber linear chirped grating 6; wherein the 980 nm pump laser diode 1, the polarization-maintaining gain fiber 3, and the polarization-maintaining fiber linear chirped grating 6 are respectively connected to the wavelength division multiplexer 2, the polarization-maintaining gain fiber 3 is connected to the polarization-maintaining fiber Fabry-Perot electrically tuned filter 4, and the polarization-maintaining fiber Fabry-Perot electrically tuned filter 4 is connected to the reflective semiconductor saturable absorber mirror 5.

[0006] In some embodiments, the 980 nm pump laser diode 1 is connected to the reflective end 21 of the wavelength division multiplexer 2, one end of the polarization-maintaining gain fiber 3 is connected to the common end 22 of the wavelength division multiplexer 2, and the other end is connected to the polarization-maintaining fiber Fabry-Perot electrically tuned filter 4. The polarization-maintaining fiber Fabry-Perot electrically tuned filter 4 is connected to the reflective semiconductor saturable absorber mirror 5, and the polarization-maintaining fiber linear chirped grating 6 is connected to the transmission end 23 of the wavelength division multiplexer 2. All components are fused together with polarization-maintaining single-mode fibers, and all fiber optic devices form a fully polarization-maintaining linear cavity between the reflective semiconductor saturable absorber mirror 5 and the polarization-maintaining fiber linear chirped grating 6.

[0007] In some embodiments, the polarization-maintaining fiber Fabry-Perot electrically tuned filter 4 includes two first and second polarization-maintaining single-mode fibers 46 and 47 with high-reflectivity coatings 41 on their end faces and their stress axes 42 and cores 43 aligned. The polarization-maintaining single-mode fibers 46 and 47 are moved in opposite directions to change the Fabry-Perot cavity length to achieve fast electrical tuning. The wavelength tuning range is 1010–1090 nm, the passband spectral width is 0.2–5 nm, and the tuning drive frequency is 0.1–20 kHz.

[0008] In some embodiments, the polarization-maintaining gain fiber 3 is a highly ytterbium-doped fiber with a core absorption efficiency greater than 200 dB / m and a length of 0.2 to 1 m.

[0009] In some embodiments, the reflective semiconductor saturable absorber mirror has a modulation depth of 10% to 20% and is fixed to the end face of a polarization-maintaining single-mode fiber.

[0010] In some embodiments, the polarization-maintaining fiber linear chirped grating has a wavelength coverage range of 1010–1090 nm, is fabricated using a femtosecond laser flexible direct scanning line writing method, the transverse scanning line direction of the femtosecond laser is perpendicular to the fast axis of the polarization-maintaining fiber, the dispersion slope is 1–6 ps / nm, and the reflectivity is 50%–80%.

[0011] In some implementations, the operating repetition frequency range of the ultrashort pulse is 20–80 MHz, and the selected operating repetition frequency variation is less than 0.05%.

[0012] In some embodiments, the time-domain width of the ultrashort pulse is 0.5 to 50 ps, ​​determined by the passband spectral width of the polarization-maintaining fiber Fabry-Perot electrically tuned filter.

[0013] In some embodiments, the output power of the ultrashort pulse seed light source is 1–10 mW.

[0014] Secondly, the present invention provides a method for adjusting a fast-tuned ultrashort pulse seed source for an ultra-stable repetition rate all-fiber nonlinear spectrometer, comprising the following steps:

[0015] Step 1: After the 980 nm pump laser diode 1 is powered on, it outputs 980 nm pump light. The pump light enters the polarization-maintaining gain fiber 3 through the 980 / 1030 nm wavelength division multiplexer 2. The polarization-maintaining gain fiber 3 absorbs the pump laser and produces spontaneous emission light with an extremely wide linewidth.

[0016] Step 2: By setting the passband wavelength of the polarization-maintaining fiber Fabry-Perot electrically tuned filter 4 with a control voltage, the desired narrowband signal light is filtered out from the spontaneously emitted light. When a single fixed control voltage is used, a single wavelength output is achieved. When a sawtooth wave control voltage is used, wavelength scanning is achieved.

[0017] Step 3: The narrowband signal light re-enters the reflective semiconductor saturable absorber mirror 5. The portion of the signal light with lower instantaneous power is absorbed, while the portion of the signal light with higher instantaneous power is reflected back into the polarization-preserving linear cavity.

[0018] Step 4: The signal light reflected back into the cavity passes through the above-mentioned devices and enters the polarization-maintaining fiber linear chirped grating 6. Narrowband signal light within the wavelength reflection range of the polarization-maintaining fiber linear chirped grating 6 is partially reflected back into the fully polarization-maintaining linear cavity at a specific position within the polarization-maintaining fiber linear chirped grating 6. The polarization-maintaining fiber linear chirped grating 6 and the reflective semiconductor saturable absorber mirror 5 form a wavelength-independent fixed-cavity long laser resonator. The polarization-maintaining gain fiber 3 provides population inversion, realizing ultra-stable laser oscillation with high repetition rate.

[0019] Step 5: The oscillating ultrashort pulse seed laser is transmitted and output from the other end of the polarization-maintaining fiber linear chirped grating 6.

[0020] Compared with the prior art, the present invention can achieve the following beneficial effects:

[0021] 1) The ultra-stable repetition rate all-fiber nonlinear spectrometer fast-tuning ultrashort pulse seed source provided by the present invention can maintain the pulse repetition rate stability in a wide wavelength tuning range without external operation, and has the advantages of self-starting and high anti-interference.

[0022] 2) The ultrashort pulse seed source adopts a full polarization-maintaining fiber linear cavity structure. The laser is continuously reflected between the reflective semiconductor saturable absorber mirror and the polarization-maintaining fiber linear chirped grating to form oscillations. Picosecond or femtosecond ultrashort pulses are generated according to the passband spectral width of the polarization-maintaining fiber Fabry-Perot electric tuned filter.

[0023] 3) Based on the intracavity fiber dispersion parameters of the ultrashort pulse seed light source, a polarization-maintaining fiber linear chirped grating is prepared to achieve passive intracavity dispersion compensation, so that light of different wavelengths has the same cavity length, thus eliminating the need for mechanical time delay operation on the fiber optical path during wavelength tuning. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the structure of the fast-tuned ultrashort pulse seed source for the ultra-stable repetition rate all-fiber nonlinear spectrometer of the present invention.

[0025] Figure 2 The image shows the output wavelength spectrum of the fast-tuned ultrashort pulse seed source of the ultra-stable repetition rate all-fiber nonlinear spectrometer of the present invention during tuning.

[0026] Figure 3 The diagram shows the output pulse repetition frequency of the fast-tuned ultrashort pulse seed source of the ultra-stable repetition rate all-fiber nonlinear spectrometer of the present invention at different wavelengths.

[0027] Figure 4 This is a structural diagram of the polarization-maintaining fiber Fabry-Perot electrically tuned filter for the fast-tuned ultrashort pulse seed source of the ultra-stable repetition rate all-fiber nonlinear spectrometer of the present invention.

[0028] Figure 5 This is a flowchart illustrating the adjustment method of the fast-tuned ultrashort pulse seed source for the ultra-stable repetition rate all-fiber nonlinear spectrometer of the present invention. Detailed Implementation

[0029] To more clearly illustrate the present invention, the technical solution proposed by the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] Example 1, such as Figure 1 As shown, the fast-tuned ultrashort pulse seed source for the ultra-stable repetition rate all-fiber nonlinear spectrometer in this embodiment includes a 980nm pump laser diode 1, a wavelength division multiplexer 2 and its reflection end 21, common end 22, transmission end 23, polarization-maintaining gain fiber 3, a polarization-maintaining fiber Fabry-Perot electrically tuned filter 4, a reflective semiconductor saturable absorber mirror 5, and a polarization-maintaining fiber linear chirped grating 6. 980nm The nm pump laser diode 1 is connected to the reflection end 21 of the wavelength division multiplexer 2. One end of the polarization-maintaining gain fiber 3 is connected to the common end 22 of the wavelength division multiplexer 2, and the other end is connected to the polarization-maintaining fiber Fabry-Perot electrically tuned filter 4. The polarization-maintaining fiber Fabry-Perot electrically tuned filter 4 is connected to the reflective semiconductor saturable absorber mirror 5. The polarization-maintaining fiber linear chirped grating 6 is connected to the transmission end 23 of the wavelength division multiplexer 2. All components are fused together with polarization-maintaining single-mode fibers. All fiber devices form a fully polarization-maintaining linear cavity between the reflective semiconductor saturable absorber mirror 5 and the polarization-maintaining fiber linear chirped grating 6.

[0031] In practical operation, after the 980 nm pump laser diode 1 is powered on, it outputs 980 nm. Pump light of nm is fed into a polarization-maintaining gain fiber 3 via a wavelength division multiplexer 2. The polarization-maintaining gain fiber 3 absorbs the pump laser and produces spontaneous emission light with extremely wide linewidth. In the polarization-maintaining fiber Fabry-Perot electrically tuned filter 4, the piezoelectric ceramic 45 and the auxiliary clamping device 44 drive the two polarization-maintaining fibers to move in opposite directions or change the Fabry-Perot cavity length to achieve rapid electrical tuning. The desired narrowband signal light is filtered out from the spontaneous emission light. The narrowband signal light then enters the reflective semiconductor saturable absorber mirror 5. The portion of the signal light with lower instantaneous power is absorbed, and the portion with higher instantaneous power is reflected back into the cavity. After passing through the above devices in sequence, it enters the polarization-maintaining fiber linear chirped grating 6. The narrowband signal light within the wavelength reflection range of the polarization-maintaining fiber linear chirped grating is partially reflected back into the linear cavity at a specific position within the polarization-maintaining fiber linear chirped grating to form a laser resonant cavity. The signal light that is not reflected back into the linear cavity is output from the other end of the polarization-maintaining fiber linear chirped grating. When the pump power increases to exceed the cavity loss, the laser travels back and forth multiple times within the cavity to meet the mode-locking oscillation condition, and finally a femtosecond or picosecond pulse is transmitted and output from the polarization-maintaining fiber linear chirped grating 6.

[0032] The pulse repetition rate of an ultrashort pulse seed source without polarization-maintaining fiber linear chirped gratings will change with intracavity dispersion during wavelength tuning, such as... Figure 3 The solid lines in the diagram show that longer wavelengths correspond to higher pulse repetition frequencies. Since different wavelengths of light have different optical path lengths within a polarization-maintaining fiber linear chirped grating, precise design of the dispersion parameters of the polarization-maintaining fiber linear chirped grating is crucial. Figure 3 The solid line yields a value of 1.33 ps / nm. At this value, the intracavity dispersion of the ultrashort pulse seed source is compensated, achieving a wavelength-independent fixed cavity length, thus ensuring consistent repetition rates for ultrashort pulses at various wavelengths. Figure 3 As shown by the dashed line, no mechanical delay operation is required on the optical fiber path during wavelength tuning.

[0033] Specifically, the polarization-maintaining gain fiber 3 is a highly ytterbium-doped fiber with a core absorption efficiency greater than 200 dB / m and a length of 0.2–1 m.

[0034] Specifically, the polarization-maintaining fiber Fabry-Perot electrically tuned filter 4 uses two polarization-maintaining single-mode fibers with high-reflectivity coatings 41 on their end faces and aligned stress axes 42 and fiber cores 43. The two polarization-maintaining fibers are moved in opposite directions by a piezoelectric ceramic 45 and an auxiliary clamping device 44 to change the Fabry-Perot cavity length and achieve rapid electrical tuning. The wavelength tuning range is 1010–1090 nm, the passband spectral width is 0.2–5 nm, and the tuning drive frequency is 0.1–20 kHz.

[0035] Specifically, the reflective semiconductor saturable absorber 5 has a modulation depth of 10% to 20% and is fixed to the end face of a polarization-maintaining single-mode fiber.

[0036] Specifically, the polarization-maintaining fiber linear chirped grating has a wavelength coverage range of 1010–1090 nm, is fabricated using a femtosecond laser flexible direct scanning line writing method, the transverse scanning line direction of the femtosecond laser is perpendicular to the fast axis of the polarization-maintaining fiber, the dispersion slope is 1–6 ps / nm, and the reflectivity is 50%–80%.

[0037] Specifically, the operating repetition frequency range of the ultrashort pulse is 20–80 MHz, and the selected operating repetition frequency variation is less than 0.05%.

[0038] Specifically, the time-domain width of the ultrashort pulse is 0.5–50 ps, ​​which is determined by the passband spectral width of the polarization-maintaining fiber Fabry-Perot electrically tuned filter.

[0039] Specifically, the output power of the ultrashort pulse seed light source is 1 to 10 mW.

[0040] like Figure 4 As shown, the polarization-maintaining fiber Fabry-Perot electrically tunable filter consists of two first and second polarization-maintaining single-mode fibers 46 and 47, with high-reflectivity coatings 41 on their end faces and their stress axes 42 and cores 43 aligned. Rapid electrical tuning is achieved by moving the two polarization-maintaining fibers in opposite directions via a piezoelectric ceramic 45 and an auxiliary clamping device 44, thus changing the Fabry-Perot cavity length. The wavelength tuning range is 10¹⁰–10⁹⁰ nm, the passband width is 0.2–5 nm, and the tuning drive frequency is 0.1–20 kHz. The reflective semiconductor saturable absorber mirror 5 is fixed to the polarization-maintaining fiber Fabry-Perot electrically tunable filter 4.

[0041] Example 2, as follows Figure 5 The diagram shows the overall flow of a fast-tuned ultrashort pulse seed source adjustment method for an ultra-stable repetition rate all-fiber nonlinear spectrometer according to the present invention, including the following steps:

[0042] Step 1: After the 980 nm pump laser diode 1 is powered on, it outputs 980 nm pump light. The pump light enters the polarization-maintaining gain fiber 3 through the 980 / 1030 nm wavelength division multiplexer 2. The polarization-maintaining gain fiber 3 absorbs the pump laser and produces spontaneous emission light with an extremely wide linewidth.

[0043] Step 2: By setting the passband wavelength of the polarization-maintaining fiber Fabry-Perot electrically tuned filter 4 with a control voltage, the desired narrowband signal light is filtered out from the spontaneously emitted light. When a single fixed control voltage is used, a single wavelength output is achieved. When a sawtooth wave control voltage is used, wavelength scanning is achieved.

[0044] Step 3: The narrowband signal light re-enters the reflective semiconductor saturable absorber mirror 5. The portion of the signal light with lower instantaneous power is absorbed, while the portion of the signal light with higher instantaneous power is reflected back into the polarization-preserving linear cavity.

[0045] Step 4: The signal light reflected back into the cavity passes through the above-mentioned devices and enters the polarization-maintaining fiber linear chirped grating 6. Narrowband signal light within the wavelength reflection range of the polarization-maintaining fiber linear chirped grating 6 is partially reflected back into the fully polarization-maintaining linear cavity at a specific position within the polarization-maintaining fiber linear chirped grating 6. The polarization-maintaining fiber linear chirped grating 6 and the reflective semiconductor saturable absorber mirror 5 form a wavelength-independent fixed-cavity long laser resonator. The polarization-maintaining gain fiber 3 provides population inversion, realizing ultra-stable laser oscillation with high repetition rate.

[0046] Step 5: The oscillating ultrashort pulse seed laser is transmitted and output from the other end of the polarization-maintaining fiber linear chirped grating 6.

[0047] The above description is merely 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. The above description is merely a preferred embodiment of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered to fall within the protection scope of the present invention.

Claims

1. A fast-tuning ultrashort pulse seed source tuning system for an ultra-stable repetition rate all-fiber nonlinear spectrometer, characterized in that, The system includes a fully polarization-maintaining linear cavity, which further includes a 980 nm pump laser diode (1), a wavelength division multiplexer (2) and its reflecting end (21), common end (22), transmission end (23), polarization-maintaining gain fiber (3), a polarization-maintaining fiber Fabry-Perot electrically tuned filter (4), a reflective semiconductor saturable absorber mirror (5), and a polarization-maintaining fiber linear chirped grating (6); wherein the 980 nm pump laser diode (1), the polarization-maintaining gain fiber (3), and the polarization-maintaining fiber linear chirped grating (6) are respectively connected to the wavelength division multiplexer (2), the polarization-maintaining gain fiber (3) is connected to the polarization-maintaining fiber Fabry-Perot electrically tuned filter (4), and the polarization-maintaining fiber Fabry-Perot electrically tuned filter (4) is connected to the reflective semiconductor saturable absorber mirror (5); its tuning method includes the following steps: Step 1: After the 980nm pump laser diode (1) is powered on, it outputs 980 nm pump light. The pump light enters the polarization-maintaining gain fiber (3) through the 980 / 1030nm wavelength division multiplexer (2). The polarization-maintaining gain fiber (3) absorbs the pump laser and produces spontaneous emission light with an extremely wide linewidth. Step 2: Set the passband wavelength of the polarization-maintaining fiber Fabry-Perot electrically tuned filter (4) by controlling the voltage to filter out the required wavelength of narrowband signal light from the spontaneous emission light. When a single fixed control voltage is used, it is a single wavelength output. When a sawtooth wave control voltage is used, wavelength scanning is achieved. Step 3: The narrowband signal light re-enters the reflective semiconductor saturable absorber mirror (5). The portion of the signal light with lower instantaneous power is absorbed, and the portion of the signal light with higher instantaneous power is reflected back into the fully polarization-maintaining linear cavity. The ultrashort pulse seed source adopts a fully polarization-maintaining fiber linear cavity structure. The laser is continuously reflected between the reflective semiconductor saturable absorber mirror and the polarization-maintaining fiber linear chirped grating to form oscillation. Picosecond or femtosecond ultrashort pulses are generated according to the passband spectral width of the polarization-maintaining fiber Fabry-Perot electrically tuned filter, realizing passive compensation of intracavity dispersion, so that light of different wavelengths has the same cavity length. The intracavity dispersion of the ultrashort pulse seed source is compensated, realizing a wavelength-independent fixed cavity length, so that the repetition frequency of ultrashort pulses at each wavelength remains consistent. Step 4: The narrowband signal light reflected back into the cavity passes sequentially through the wavelength division multiplexer (2), the polarization-maintaining gain fiber (3), the polarization-maintaining fiber Fabry-Perot electrically tuned filter (4), and the reflective semiconductor saturable absorber (5). After being reflected by the reflective semiconductor saturable absorber (5), it enters the polarization-maintaining fiber linear chirped grating (6). The narrowband signal light within the wavelength reflection range of the polarization-maintaining fiber linear chirped grating (6) is partially reflected back into the fully polarization-maintaining linear cavity at a specific position within the polarization-maintaining fiber linear chirped grating. The polarization-maintaining fiber linear chirped grating (6) and the reflective semiconductor saturable absorber (5) form a wavelength-independent fixed-cavity long laser resonator. The polarization-maintaining gain fiber (3) provides population inversion, realizing ultra-stable repetition rate laser oscillation. Step 5: The oscillating ultrashort pulse seed laser is transmitted and output from the other end of the polarization-maintaining fiber linear chirped grating (6).

2. The ultra-stable repetition rate all-fiber nonlinear spectrometer fast-tuning ultrashort pulse seed source tuning system according to claim 1, characterized in that, The 980 nm pump laser diode (1) is connected to the reflection end (21) of the wavelength division multiplexer (2). One end of the polarization-maintaining gain fiber (3) is connected to the common end (22) of the wavelength division multiplexer (2), and the other end is connected to the polarization-maintaining fiber Fabry-Perot electrically tuned filter (4). The polarization-maintaining fiber Fabry-Perot electrically tuned filter (4) is connected to the reflective semiconductor saturable absorber mirror (5). The polarization-maintaining fiber linear chirped grating (6) is connected to the transmission end (23) of the wavelength division multiplexer (2). All components are fused together with polarization-maintaining single-mode fiber. All fiber devices form a fully polarization-maintaining linear cavity between the reflective semiconductor saturable absorber mirror (5) and the polarization-maintaining fiber linear chirped grating (6).

3. The ultra-stable repetition rate all-fiber nonlinear spectrometer fast-tuning ultrashort pulse seed source tuning system according to claim 1, characterized in that, The polarization-maintaining fiber Fabry-Perot electrically tuned filter (4) includes two polarization-maintaining single-mode fibers (46 and 47) with high-reflectivity films (41) on their end faces and their stress axes (42) and fiber cores (43) aligned. The first polarization-maintaining single-mode fiber (46) and the second polarization-maintaining single-mode fiber (47) move in opposite directions to change the Fabry-Perot cavity length and achieve fast electrical tuning. The wavelength tuning range is 1010 to 1090 nm, the passband spectral width is 0.2 to 5 nm, and the tuning drive frequency is 0.1 to 20 kHz.

4. The ultra-stable repetition rate all-fiber nonlinear spectrometer fast-tuning ultrashort pulse seed source tuning system according to claim 1, characterized in that, The reflective semiconductor saturable absorber mirror has a modulation depth of 10% to 20% and is fixed to the end face of a polarization-maintaining single-mode fiber.

5. The ultra-stable repetition rate all-fiber nonlinear spectrometer fast-tuning ultrashort pulse seed source tuning system according to claim 1, characterized in that, The polarization-maintaining fiber linear chirped grating has a wavelength coverage range of 1010–1090 nm and is fabricated using a femtosecond laser flexible direct scanning line writing method. The transverse scanning line direction of the femtosecond laser is perpendicular to the fast axis of the polarization-maintaining fiber, the dispersion slope is 1–6 ps / nm, and the reflectivity is 50%–80%.

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