Passive mode-locking fiber laser based on nano-grating-based polarizer and working method of passive mode-locking fiber laser

Abstract

The invention relates to a passive mode-locking fiber laser based on a nanometer grating-based polarizer and a working method thereof. The passive mode-locking fiber laser comprises a pumping source, a wavelength division multiplexer, two polarization controllers, two nanometer grating lines, a single-mode optical coupler, a polarization independent isolator and a section of gain fiber. The pumping source provides 976nm pumping light for the annular cavity; the wavelength division multiplexer is used for inputting 976nm pump light into the annular cavity; the two nano grating lines play an equivalent role of a block polarizer, and the two nano grating lines and the two polarization controllers jointly realize an NPR effect; the single-mode optical fiber coupler is used for coupling out 10% of resonance light in the cavity and taking the resonance light as output light of the laser; the polarization independent isolator is used for ensuring that the resonant light runs in the annular cavity in a one-way manner; the section of gain optical fiber is used for converting 976nm pump light into 1032m resonant light; the invention is realized for the first time, belongs to a new laser system, and plays a very important role in subsequent research and development.

Description

Technical Field

[0001] The present invention belongs to the technical field of fiber lasers, and particularly relates to a passively mode-locked fiber laser based on a nano-grating polarizer and a working method thereof. Background Art

[0002] Passively mode-locked fiber lasers have evolved from basic science to commercial instruments and are widely used in telecommunications, optical frequency combs, metrology, microscopy, and nonlinear science. In the past two decades, there have been various methods for realizing passively mode-locked fiber lasers, mainly including nonlinear polarization rotation (NPR), nonlinear interferometry, semiconductor saturation absorber mirror (SESAM), and single-walled carbon nanotubes. Among them, NPR is the most commonly used method. Due to the optical Kerr effect, polarization changes related to the pulse intensity will occur. When light is coupled from the fiber into the polarizer, the transmittance through the polarizer is intensity-dependent. By appropriately adjusting the polarization controller to control the polarization state of the light and ensuring that the central intensity of the pulse has the highest transmittance, a saturable absorber is formed. Usually, a bulk optical polarizer is used in this scheme. In contrast, fiber polarizers with light weight and high coupling efficiency have more advantages. The 45-degree tilted grating-based polarizer is one of the current representatives of fiber polarizers. It has strong polarization-dependent loss (PDL) and has been used in some passively mode-locked fiber lasers. The 45-degree tilted grating-based polarizer is composed of a 45-degree tilted fiber grating. As Figure 1 shown, in principle, when a light pulse passes through such a grating, the transmission loss of the p-polarized light (the light with the polarization direction parallel to the transmission plane) is very small, while the transmission loss of the s-polarized light (the light with the polarization direction perpendicular to the transmission plane) is very large. Most of the 45-degree tilted gratings are fabricated by the method of ultraviolet light + phase mask. In addition, an in-fiber straight waveguide is also used in passively mode-locked fiber lasers. The in-fiber straight waveguide is a waveguide with refractive index modulation inscribed in the core and cladding of a single-mode fiber. As Figure 2 shown, the waveguide length is 6 mm. Theoretically, the offset can be set to any non-zero value, and the offset position cannot exceed the core range. The in-fiber straight waveguide has slight polarization-dependent loss and has the advantages of low cost, stable mechanical properties, low loss, and simple structure.

[0003] The core of a passively mode-locked fiber laser based on a 45-degree tilted grating polarizer is the 45-degree tilted grating polarizer. Although the 45-degree tilted grating has strong polarization-dependent loss, light weight, and high coupling efficiency, the manufacturing difficulty of the 45-degree tilted grating is relatively high. It requires an expensive phase mask and an ultraviolet light source to be used in combination. If the 45-degree tilted fiber grating is fabricated by femtosecond laser direct writing, the accuracy of line-by-line direct writing needs to be considered. For example, Figure 1 the 45-degree angle in Figure 1 needs to be strictly controlled. The longer the grating length, the more periods there are, and the higher the consistency requirement for each direct writing line. The in-fiber straight waveguide has weak polarization-dependent loss, low cost, stable mechanical properties, low loss, and simple structure. Its principle is to generate nonlinear multimode interference and excite the nonlinear polarization rotation effect. However, for this solution, the writing length is 6 mm, the writing position cannot exceed the core range, and the most crucial point is that the axis of the straight waveguide must have a non-zero offset difference from the axis of the fiber core. Therefore, the selection of the offset difference is a complex process. Summary of the Invention

[0004] To overcome the above problems existing in the prior art, the present invention proposes a passively mode-locked fiber laser based on a nano-grating polarizer;

[0005] The present invention also provides a working method for the above passively mode-locked fiber laser.

[0006] The technical solution of the present invention is as follows:

[0007] A passively mode-locked fiber laser based on a nano-grating polarizer includes:

[0008] A pump source (Pump), a wavelength division multiplexer (WDM), two polarization controllers, two nano-grating lines, a single-mode optical coupler (Optical coupler, OC), a polarization-independent isolator (PI-ISO), and a section of gain fiber (Ytterbium-doped fiber, YDF);

[0009] The two nano-grating lines are arranged between the two polarization controllers;

[0010] The pump source provides 976 nm pump light for the ring cavity; the wavelength division multiplexer inputs the 976 nm pump light into the ring cavity; the two nano-grating lines play the equivalent role of a bulk polarizer and jointly with the two polarization controllers achieve the NPR effect; the single-mode fiber coupler couples out 10% of the resonant light in the cavity and serves as the output light of the laser; the polarization-independent isolator is used to ensure the unidirectional operation of the resonant light in the ring cavity; a section of gain fiber converts the 976 nm pump light into 1032 m resonant light.

[0011] Preferably according to the present invention, the lengths of the two nano-grating lines are 1-2 mm, and the spacing lengths of the two nano-grating lines are 1-25 cm.

[0012] The length of the nano-grating and the scattering loss are theoretically positively correlated, that is, the longer the length, the greater the scattering loss. The spacing length theoretically has no relation with the magnitude of the scattering loss, but the size of the spacing will affect the integrated length of the nano-grating device, that is, the larger the spacing, the larger the integrated length.

[0013] Further preferably, the lengths of the two nano-grating lines are 1 mm, and the spacing lengths of the two nano-grating lines are 25 cm.

[0014] Preferably according to the present invention, the method for obtaining the two nano-grating lines includes: directly writing using an ultrafast laser at the core center position, with the polarization direction of the ultrafast laser perpendicular to the core axis, and directly writing the two nano-grating lines.

[0015] Preferably according to the present invention, the length of the gain fiber is 0.33-1 m, and the total length of the ring cavity is 5-40 m.

[0016] Further preferably, the length of the gain fiber is 0.33 m, and the total length of the ring cavity is 8.3 m.

[0017] The working method of the above passive mode-locked fiber laser includes:

[0018] First, a 976 nm pump source outputs pump light, which enters the gain fiber through a wavelength division multiplexer. After the gain fiber absorbs the pump light, spontaneous emission light is generated, which contains 1032 nm signal light. Only the 1032 nm signal light is amplified in the ring cavity. At this time, the polarization controller (PC2) controls the polarization state of the signal light, and by controlling the polarization controller (PC2), the polarization state of the signal light is made into an elliptical polarization state;

[0019] Then, control the polarization controller (PC1) to make the central part of the signal light pulse have the highest transmittance when passing through the nano-grating line. At this time, through continuous circulation in the ring cavity, a quasi-noise pulse mode-locking is obtained;

[0020] Finally, through a single-mode fiber coupler, 10% of the energy in the ring cavity is output.

[0021] The beneficial effects of the present invention are:

[0022] The passive mode-locked fiber laser based on a nano-grating polarizer proposed by the present invention, firstly, includes a core device directly written by femtosecond laser: a nano-grating polarizer. The nano-grating polarizer can be composed of 2 engraved lines, with a distance of 25 cm between them, and the length of a single engraved line is 1 mm. The position of the engraved line coincides with the axis of the fiber core, which simplifies the selection problem of the deviation amount of the engraved line position. Secondly, the arrangement of the engraved line positions is along the axis direction of the fiber core, which belongs to a new arrangement method. This method can reduce the difficulty of direct writing, and the positions of the front and rear engraved lines can be flexibly adjusted. Thirdly, the nano-grating contained in the engraved line is the core to achieve mode locking. After adjusting the laser parameters and scanning parameters, the nano-grating polarizer can be mass-produced and fused, which simplifies the generation process. Finally, the passive mode-locked laser based on the nano-grating polarizer is realized for the first time, belonging to a new laser system, which will play a very important role in subsequent research and development. Brief Description of the Drawings

[0023] Figure 1 It is a schematic structural diagram of a 45-degree tilted grating polarizer;

[0024] Figure 2 It is a schematic diagram of a straight waveguide in the fiber;

[0025] Figure 3 It is a schematic diagram of a nano-grating line;

[0026] Figure 4 It is an optical path diagram of the passive mode-locked fiber laser;

[0027] Figure 5 It is a schematic diagram of the pulse sequence displayed on the oscilloscope in the mode-locked state;

[0028] Figure 6 It is a schematic diagram of the FFT spectrum displayed on the oscilloscope in the mode-locked state;

[0029] Figure 7 It is a spectrogram displayed on the spectrometer in the mode-locked state;

[0030] Figure 8 It is a pulse profile diagram displayed on the autocorrelator in the mode-locked state;

[0031] Figure 9 It is a schematic diagram of the power stability curve measured by the power meter;

[0032] Figure 10 It is a schematic diagram of the relationship curve between the pump input power and the output power of the ring cavity measured by the power meter; Detailed Embodiment

[0033] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be described in detail below in conjunction with the drawings and specific embodiments.

[0034] Example 1

[0035] A passively mode-locked fiber laser based on a nano-grating polarizer, comprising:

[0036] A pump source (Pump), a wavelength division multiplexer (WDM), two polarization controllers, two nano-grating lines, a single-mode optical coupler (Optical coupler, OC), a polarization-independent isolator (PI-ISO), and a section of gain fiber (Ytterbium-doped fiber, YDF);

[0037] The two nano-grating lines are arranged between the two polarization controllers; Figure 4 Two directly written nano-grating lines are used in the passively mode-locked fiber laser, which is equivalent to a polarizer, and the front and rear two polarization controllers (Polarization controller, PC1 and PC2) jointly realize the NPR effect.

[0038] Figure 4 Fig. is the optical path diagram of the passively mode-locked fiber laser. The pump source provides 976nm pump light for the ring cavity; the wavelength division multiplexer inputs the 976nm pump light into the ring cavity; the two nano-grating lines play the equivalent role of a bulk polarizer and jointly with the two polarization controllers realize the NPR effect; the single-mode fiber coupler couples out 10% of the resonant light in the cavity and serves as the output light of the laser; the polarization-independent isolator is used to ensure the unidirectional operation of the resonant light in the ring cavity; a section of gain fiber converts the 976nm pump light into 1032m resonant light.

[0039] The present invention utilizes the nano-gratings in the engraved lines to realize the function of the polarizer, which is essentially different from the multimode interference of the optical fiber waveguide. The nano-gratings mainly realize the polarization-related characteristics based on the scattering loss principle. The nano-gratings utilize the scattering principle, and the number and quality of the nano-gratings determine the magnitude of the scattering loss, and the number and quality of the nano-gratings are only related to the parameters of the processing laser and the parameters of the processing technology; in addition, the processing position of the nano-gratings is fixed, only at the core position, which is relatively easy to locate; therefore, by controlling the processing parameters and process parameters, the length of the engraved lines can be effectively shortened, and the typical length of the present invention is 1mm. Secondly, the optical fiber waveguide utilizes the principle of multimode interference, and a certain length of the optical fiber waveguide is required to excite an obvious multimode interference effect. The typical length of the existing invention is 6mm; in addition, the position of the optical fiber waveguide also has a great influence, and at different positions, the intensity of the multimode interference generated will also vary greatly. Therefore, it is not easy to effectively determine the appropriate position of the optical fiber waveguide.

[0040] Example 2

[0041] The passively mode-locked fiber laser based on the nano-grating polarizer according to Example 1 is characterized in that:

[0042] The lengths of the two nano-grating lines are 1 - 2 mm, and the spacing lengths between the two nano-grating lines are 1 - 25 cm.

[0043] The length of the nano-grating and the scattering loss are theoretically positively correlated, that is, the longer the length, the greater the scattering loss. The spacing length theoretically has no relation with the magnitude of the scattering loss, but the size of the spacing will affect the integrated length of the nano-grating device, that is, the larger the spacing, the larger the integrated length.

[0044] The length of a single nano-grating line in the present invention is only 1 mm, without the need for continuous direct writing of a 6-mm length. The distance is shortened by 6 times, and the scribing time is shortened.

[0045] The present invention uses the nano-grating in scribing to realize the function of a polarizer, which is essentially different from the multimode interference of an optical fiber waveguide. The nano-grating mainly realizes polarization-related characteristics based on the principle of scattering loss. The nano-grating utilizes the scattering principle, and the number and quality of the nano-gratings determine the magnitude of the scattering loss. The number and quality of the nano-gratings are only related to the parameters of the processing laser and the parameters of the processing technology. In addition, the processing position of the nano-grating is fixed, only at the core position, which is relatively easy to locate. Therefore, by controlling the processing parameters and process parameters, the scribing length can be effectively shortened, and the typical length of the present invention is 1 mm. Secondly, the optical fiber waveguide utilizes the principle of multimode interference, and a certain length of optical fiber waveguide is required to excite an obvious multimode interference effect. The typical length of the existing invention is 6 mm. In addition, the position of the optical fiber waveguide also has a great influence. At different positions, the intensity of the multimode interference generated will also vary greatly. Therefore, it is not easy to effectively determine the appropriate position of the optical fiber waveguide.

[0046] The methods for obtaining the two nano-grating lines include: as Figure 3 shown, use an ultrafast laser to perform direct writing at the center position of the core. The polarization direction of the ultrafast laser is perpendicular to the core axis, and two nano-grating lines are directly written. The width of the nano-grating line is related to the diameter of the focused spot. In actual situations, the spacing length can be flexibly adjusted. However, not all scribing will produce nano-gratings. Therefore, specific laser scanning speeds and specific ultrafast laser parameters are required. The generated nano-gratings need to be measured with an orthogonal polarization light source. When it is determined that the nano-gratings are generated, this scribing line will have polarization-related loss. Generally, the polarization-related loss is less than the insertion loss. If the polarization-related loss is to be greater than the insertion loss, specific laser parameters need to be set.

[0047] The scribing position of the present invention coincides with the core axis, and the scribing lines can be arranged along the core axis. The position is fixed, avoiding the control of the scribing deviation amount and reducing the scribing complexity.

[0048] The length of the gain fiber is 0.33 - 1 m, and the total length of the ring cavity is 5 - 40 m.

[0049] Example 3

[0050] The passively mode-locked fiber laser based on a nano-grating polarizer according to Example 2 is different in that:

[0051] According to Figure 4 the optical path diagram of, a laser system was built. In the experiment, the length of the gain fiber YDF used was 0.33 m, the interval length of the nano-grating polarizer was 25 cm, and the remaining fibers were single-mode fibers (fiber type: HI1060, CORNING). The total length of the ring cavity was 8.3 m. In the experiment, the polarization-dependent loss of the nano-grating was 3.2 dB, and the insertion loss was less than 10 dB. To achieve mode locking, a pump light with an input wavelength of 976 nm and an average power of 783 mW was input. By adjusting two PCs, the laser system could achieve stable mode locking. The mode-locking results were as follows: Figure 5 is a schematic diagram of the pulse sequence shown on the oscilloscope in the mode-locked state; Figure 6 is a schematic diagram of the FFT spectrum shown on the oscilloscope in the mode-locked state; Figure 5 In, the abscissa is time and the ordinate is the normalized voltage value. Figure 5 In, the detected mode-locked pulse signal sequence on the oscilloscope, and the interval between adjacent pulse signals is 40 ns; Figure 6 In, the abscissa is the repetition frequency and the ordinate is the intensity value. Figure 6 In, after the oscilloscope pulse sequence undergoes FFT transformation, the fundamental frequency is 24.09 MHz, the measured set resolution bandwidth is 1.46 kHz, the range is 458 kHz, the signal-to-noise ratio of the fundamental frequency and the adjacent frequencies is 49 dB, and the signal-to-noise ratio of the fundamental frequency and the background noise is 51 dB. The oscilloscope models used were ROHDE & SCHWARZ RTB2004 2.5 GSa / s and 5 GSa / s respectively;

[0052] Figure 7 is the spectrogram shown on the spectrometer in the mode-locked state; Figure 7 In, the abscissa is the wavelength and the ordinate is the intensity value. Figure 7 In, the spectral center wavelength of the output light is 1032 nm, and the 3 dB spectral bandwidth is 2.5 nm. The spectrometer model used was YOKOGAWA AQ6370D;

[0053] Figure 8 is the pulse profile diagram shown on the autocorrelator in the mode-locked state; Figure 8 In, the abscissa is time and the ordinate is the intensity value. Figure 8In it, when the scanning range is 150 ps, the pulse is a sharp straight line. When the scanning range is 15 ps (inset), the fitted pulse width of the peak is 220 fs, which is very consistent with the typical characteristics of noise-like pulses. The autocorrelator model used is APE Pulse Check 2.2.103;

[0054] Figure 9 It is a schematic diagram of the power stability curve measured by a power meter; Figure 9 In it, the abscissa is time and the ordinate is the output power. Figure 9 In it, the output power was monitored for 3 hours. The root mean square of the output power within 3 hours is 0.53%. From the inset, within 3 hours, the maximum power fluctuation is 0.02 mW. The power meter and probe models used are Thorlabs PM100D & S145C; Figure 10 It is a schematic diagram of the relationship curve between the pump input power and the output power of the ring cavity measured by a power meter;

[0055] Figure 10 In it, the abscissa is the pump power and the ordinate is the output power. Figure 10 In it, the experimental data of 3 points are selected and then fitted with a straight line. The slope of the straight line is 0.00141, indicating that the loss in the ring cavity is very large and needs to be optimized later. The power meter and probe models used are Thorlabs PM100D & S145C.

[0056] The specific experimental results are as follows: Pulse sequence and FFT spectrum: Pulse interval 40 ns, signal-to-noise ratio 49 dB, fundamental frequency 24.09 MHz; Spectrum: Resolution 0.02 nm, bandwidth 14 nm, central wavelength 1032 nm; Autocorrelation: noise-like pulse. When the scanning range is 150 ps, the pulse width cannot be measured; when the scanning range is 15 ps, the measured pulse width is 220 fs; Power stability: Measurement time > 3 h, RMS is 0.53%; Relationship between pump power and output power: 3 points are selected, and the slope of the fitted straight line is 0.001407.

[0057] Example 4

[0058] The working method of the passively mode-locked fiber laser according to any one of Examples 1-3 includes:

[0059] First, a 976 nm pump source outputs pump light, which enters the gain fiber through a wavelength division multiplexer. After the gain fiber absorbs the pump light, spontaneous emission light is generated, which contains 1032 nm signal light. Due to the unidirectional isolation function of the polarization-independent isolator, the generated spontaneous emission light can only follow Figure 4Circular amplification is carried out in the clockwise direction in the optical path. Due to the balance relationship between the loss and gain of the ring cavity, finally only the signal light at 1032 nm is amplified in the ring cavity. At this time, the polarization controller (PC2) controls the polarization state of the signal light, and by controlling the polarization controller (PC2), the polarization state of the signal light is changed into an elliptical polarization state;

[0060] Then, control the polarization controller (PC1) to make the central part of the signal light pulse have the highest transmittance when passing through the nano-grating lines. At this time, through continuous circulation in the ring cavity, finally Figure 7 the noise-like pulse mode locking as shown is obtained;

[0061] Finally, through the single-mode fiber coupler, 10% of the energy in the ring cavity is output.

Claims

1. A passive mode-locked fiber laser based on a nano-grating polarizer, characterized in that, Comprising: A pump source, a wavelength division multiplexer, two polarization controllers, two nano-grating lines, a single-mode optical coupler, a polarization-independent isolator, and a section of gain fiber; The two nano-grating lines are arranged between the two polarization controllers; The pump source provides 976 nm pump light for the ring cavity; the wavelength division multiplexer inputs the 976 nm pump light into the ring cavity; The two nano-grating lines play the equivalent role of a bulk polarizer and together with the two polarization controllers achieve the NPR effect; the single-mode fiber coupler couples out 10% of the resonant light in the cavity and serves as the output light of the laser; the polarization-independent isolator is used to ensure the unidirectional operation of the resonant light in the ring cavity; A section of gain fiber converts the 976 nm pump light into 1032 nm resonant light.

2. The passive mode-locked fiber laser based on a nano-grating polarizer according to claim 1, characterized in that, The lengths of the two nano-grating lines are 1 - 2 mm, and the distance between the two nano-grating lines is 1 - 25 cm.

3. The passive mode-locked fiber laser based on a nano-grating polarizer according to claim 1, wherein The lengths of the two nano-grating lines are 1 mm, and the distance between the two nano-grating lines is 25 cm.

4. The passive mode-locked fiber laser based on a nano-grating polarizer according to claim 1, characterized in that, The method for obtaining the two nano-grating lines includes: using an ultrafast laser for direct writing at the center position of the fiber core, with the polarization direction of the ultrafast laser perpendicular to the fiber core axis, to directly write the two nano-grating lines.

5. The passive mode-locked fiber laser based on a nano-grating polarizer according to claim 1, characterized in that The length of the gain fiber is 0.33 - 1 m, and the total length of the ring cavity is 5 - 40 m.

6. The passive mode-locked fiber laser based on a nano-grating polarizer according to claim 1, wherein The length of the gain fiber is 0.33 m, and the total length of the ring cavity is 8.3 m.

7. The working method of the passively mode-locked fiber laser according to any one of claims 1-6, characterized in that, Comprising: First, the 976 nm pump source outputs pump light, which enters the gain fiber through the wavelength division multiplexer. After the gain fiber absorbs the pump light, spontaneous emission light is generated, which contains 1032 nm signal light. Only the 1032 nm signal light is amplified in the ring cavity. At this time, the polarization controller controls the polarization state of the signal light, and by controlling the polarization controller, the polarization state of the signal light is made into an elliptical polarization state; Then, control the polarization controller to make the central part of the signal light pulse have the highest transmittance when passing through the nano-grating line. At this time, through continuous circulation in the ring cavity, noise-like pulse mode locking is obtained; Finally, through the single-mode fiber coupler, 10% of the energy in the ring cavity is output.

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