Passive Mode-Locked Fiber Laser Based on Nanograting Polarizer and Its Working Method

Abstract

This invention relates to a passively mode-locked fiber laser based on a nanograting-based polarizer and its operating method, comprising: a pump source, a wavelength division multiplexer, two polarization controllers, two nanograting lines, a single-mode optical coupler, a polarization-independent isolator, and a section of gain fiber; the pump source provides 976nm pump light to the ring cavity; the wavelength division multiplexer inputs the 976nm pump light into the ring cavity; the two nanograting lines act as an equivalent 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 within the cavity and serves as the laser's output light; the polarization-independent isolator ensures unidirectional operation of the resonant light within the ring cavity; and a section of gain fiber converts the 976nm pump light into 1032nm resonant light. This invention is the first of its kind and represents a novel laser system, which will play a very important role in subsequent research and development.

Description

Technical Field

[0001] This invention belongs to the field of fiber laser technology, specifically relating to a passive mode-locked fiber laser based on a nanograting polarizer and its operating method. Background Technology

[0002] Passive 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. Over the past two decades, various methods have been developed to realize passive mode-locked fiber lasers, primarily including nonlinear polarization rotation (NPR), nonlinear interferometry, semiconductor saturation absorber mirrors (SESAM), and single-walled carbon nanotubes. Among these, NPR is the most commonly used method. Due to the optical Kerr effect, a polarization change related to pulse intensity occurs. When light is coupled from the fiber to the polarizer, the transmittance through the polarizer is intensity-dependent. By appropriately adjusting the polarization controller to control the polarization state of the light, ensuring the highest transmittance at the center of the pulse, a saturable absorber is formed. Typically, a bulk optical polarizer is used in this approach. In contrast, lightweight and highly efficient fiber polarizers offer significant advantages. The 45-degree tilted grating-based polarizer is one of the representative fiber polarizers currently available. It exhibits strong polarization dependent loss (PDL) and has been used in some passively mode-locked fiber lasers. The 45-degree tilted grating-based polarizer is constructed from a 45-degree tilted fiber grating, such as... Figure 1 As shown, in principle, when a light pulse passes through this type of grating, the transmission loss of p-light (light with polarization parallel to the transmission plane) is very small, while the transmission loss of s-light (light with polarization perpendicular to the transmission plane) is very large. 45-degree tilted gratings are mostly manufactured using ultraviolet light and a phase mask. Furthermore, straight waveguides within optical fibers are also used in passively mode-locked fiber lasers. A straight waveguide within an optical fiber is a section of refractive index-modulated waveguide etched into the core and cladding of a single-mode fiber, such as... Figure 2 As shown, the waveguide length is 6mm. Theoretically, the offset can be set to any non-zero value, and the offset position cannot exceed the fiber core range. The straight waveguide inside the fiber 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. While the 45-degree tilted grating offers advantages such as strong polarization correlation loss, light weight, and high coupling efficiency, its fabrication is challenging, requiring expensive phase masks and ultraviolet light sources. Furthermore, if a femtosecond laser direct-writing method is used to fabricate the 45-degree tilted fiber grating, the precision of line-by-line direct writing must be carefully considered. Figure 1 The 45-degree angle in the grating must be strictly controlled. The longer the grating length, the more periods there are, and the higher the consistency requirement for each straight-written line. Straight waveguides within optical fibers have weak polarization-dependent loss and offer advantages such as low cost, stable mechanical properties, low loss, and simple structure. Their principle is to generate nonlinear multimode interference, exciting a nonlinear polarization rotation effect. However, this scheme requires a 6mm writing length, the writing position cannot exceed the fiber core, and most importantly, the straight waveguide axis must maintain a non-zero offset difference from the fiber core axis. Therefore, selecting this offset difference is a complex process. Summary of the Invention

[0004] To overcome the aforementioned problems in the prior art, this invention proposes a passive mode-locked fiber laser based on a nanograting-based polarizer;

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

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

[0007] Passive mode-locked fiber lasers based on nanograting-based polarizers include:

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

[0009] Two nanograting lines are positioned between two polarization controllers;

[0010] The pump source provides 976nm pump light to the ring cavity; the wavelength division multiplexer inputs the 976nm pump light into the ring cavity; two nanograting lines act as equivalent to a block polarizer and together with two polarization controllers to achieve the NPR effect; a single-mode fiber coupler couples out 10% of the resonant light in the cavity and uses it as the output light of the laser; a polarization-independent isolator is used to ensure that the resonant light runs unidirectionally in the ring cavity; a section of gain fiber converts the 976nm pump light into 1032nm resonant light.

[0011] According to a preferred embodiment of the present invention, the length of the two nanograting lines is 1-2 mm, and the distance between the two nanograting lines is 1-25 cm.

[0012] The length of a nanograting is theoretically positively correlated with scattering loss; that is, the longer the grating, the greater the scattering loss. The spacing length is theoretically unrelated to the magnitude of scattering loss, but the spacing affects the integration length of the nanograting device; that is, the larger the spacing, the greater the integration length.

[0013] More preferably, the length of the two nanograting lines is 1 mm, and the distance between the two nanograting lines is 25 cm.

[0014] According to a preferred embodiment of the present invention, the method for obtaining two nanograting lines includes: using an ultrafast laser to perform direct writing at the center position of the fiber core, wherein the polarization direction of the ultrafast laser is perpendicular to the fiber core axis, and two nanograting lines are directly written.

[0015] According to a preferred embodiment of the present invention, the length of the gain fiber is 0.33-1m, and the total length of the annular cavity is 5-40m.

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

[0017] The operating method of the aforementioned passively mode-locked fiber laser includes:

[0018] First, the pump light output from the 976nm pump source enters the gain fiber through the wavelength division multiplexer. After the gain fiber absorbs the pump light, it generates spontaneous emission light, which contains a 1032nm signal light. Only the 1032nm signal light is amplified in the ring cavity. At this time, the polarization controller (PC2) controls the polarization state of the signal light. By controlling the polarization controller (PC2), the polarization state of the signal light is changed to an elliptic polarization state.

[0019] Then, the polarization controller (PC1) is controlled to ensure that the central part of the signal light pulse has the highest transmittance when passing through the nanograting line. At this time, the pulse is continuously circulated through the ring cavity to obtain a noise-like pulse mode-locking.

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

[0021] The beneficial effects of this invention are as follows:

[0022] This invention proposes a passively mode-locked fiber laser based on a nanograting polarizer. First, it includes a core device directly written with a femtosecond laser: a nanograting polarizer. The nanograting polarizer consists of two scribe lines, 25 cm apart, each 1 mm long, with the scribe line positions coinciding with the fiber core axis, simplifying the selection of scribe line position deviation. Second, the arrangement of the scribe lines along the fiber core axis is a novel arrangement method, reducing the difficulty of direct writing, and allowing for flexible adjustment of the positions of the two scribe lines. Third, the nanograting within the scribe lines is crucial for achieving mode-locking. After adjusting the laser and scanning parameters, the nanograting polarizer can be mass-produced and fused, simplifying the manufacturing process. Finally, this passively mode-locked laser based on a nanograting polarizer is the first of its kind, representing a novel laser system that will play a vital role in subsequent research and development. Attached Figure Description

[0023] Figure 1 A schematic diagram of a 45-degree tilted grating-based polarizer;

[0024] Figure 2 This is a schematic diagram of a straight waveguide inside an optical fiber.

[0025] Figure 3 This is a schematic diagram of a nanograting line;

[0026] Figure 4 This is the optical path diagram of a passively mode-locked fiber laser.

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

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

[0029] Figure 7 This is the spectrum displayed by the spectrometer in mode-locked mode;

[0030] Figure 8 It is the pulse profile diagram displayed by the autocorrelation instrument in mode-locked state;

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

[0032] Figure 10 This is a schematic diagram showing the relationship between the pump input power and the annular cavity output power measured by the power meter; Detailed Implementation

[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 with reference to the accompanying drawings and specific embodiments.

[0034] Example 1

[0035] Passive mode-locked fiber lasers based on nanograting-based polarizers include:

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

[0037] Two nanograting lines are positioned between two polarization controllers; Figure 4 Two directly written nanograting lines are used in a passively mode-locked fiber laser, equivalent to a polarizer, and together with two polarization controllers (PC1 and PC2) to achieve the NPR effect.

[0038] Figure 4 The diagram shows the optical path of a passively mode-locked fiber laser. The pump source provides 976nm pump light to the ring cavity; a wavelength division multiplexer inputs the 976nm pump light into the ring cavity; two nanograting lines act as equivalent to a block polarizer and, together with two polarization controllers, achieve the NPR effect; a single-mode fiber coupler couples out 10% of the resonant light in the cavity and uses it as the laser's output light; a polarization-independent isolator ensures that the resonant light travels unidirectionally within the ring cavity; and a gain fiber section converts the 976nm pump light into 1032nm resonant light.

[0039] This invention utilizes nanogratings within etched lines to function as a polarizer, fundamentally differing from multimode interference in fiber optic waveguides. Nanogratings primarily achieve polarization correlation characteristics based on the principle of scattering loss. The number and quality of nanogratings determine the magnitude of scattering loss, which in turn depends only on the parameters of the processing laser and the processing technology. Furthermore, the processing position of the nanogratings is fixed, located only at the fiber core, making them relatively easy to position. Therefore, controlling the processing and process parameters effectively shortens the etched line length; the typical length in this invention is 1 mm. Secondly, fiber optic waveguides utilize the principle of multimode interference; a certain length of fiber optic waveguide is required to elicit a significant multimode interference effect; existing inventions typically use a length of 6 mm. Additionally, the position of the fiber optic waveguide has a significant impact; different positions result in vastly different multimode interference intensities. Therefore, effectively determining the appropriate position of the fiber optic waveguide is not easy.

[0040] Example 2

[0041] The passive mode-locked fiber laser based on a nanograting polarizer described in Example 1 differs in that:

[0042] The length of the two nanograting lines is 1-2 mm, and the distance between the two nanograting lines is 1-25 cm.

[0043] The length of a nanograting is theoretically positively correlated with scattering loss; that is, the longer the grating, the greater the scattering loss. The spacing length is theoretically unrelated to the magnitude of scattering loss, but the spacing affects the integration length of the nanograting device; that is, the larger the spacing, the greater the integration length.

[0044] The present invention has a single nano-grating line with a length of only 1mm, eliminating the need for continuous direct writing of 6mm length, reducing the distance by 6 times and shortening the scribing time.

[0045] This invention utilizes nanogratings within etched lines to function as a polarizer, fundamentally differing from multimode interference in fiber optic waveguides. Nanogratings primarily achieve polarization correlation characteristics based on the principle of scattering loss. The number and quality of nanogratings determine the magnitude of scattering loss, which in turn depends only on the parameters of the processing laser and the processing technology. Furthermore, the processing position of the nanogratings is fixed, located only at the fiber core, making them relatively easy to position. Therefore, controlling the processing and process parameters effectively shortens the etched line length; the typical length in this invention is 1 mm. Secondly, fiber optic waveguides utilize the principle of multimode interference; a certain length of fiber optic waveguide is required to elicit a significant multimode interference effect; existing inventions typically use a length of 6 mm. Additionally, the position of the fiber optic waveguide has a significant impact; different positions result in vastly different multimode interference intensities. Therefore, effectively determining the appropriate position of the fiber optic waveguide is not easy.

[0046] Two methods for obtaining nanograting lines include: Figure 3 As shown, an ultrafast laser is used to directly write two nanograting lines at the center of the fiber core, with the ultrafast laser polarization direction perpendicular to the fiber core axis. The width of the nanograting lines is related to the diameter of the focused spot; in practice, the spacing can be flexibly adjusted. However, not all lines will produce nanogratings; therefore, specific laser scanning speeds and ultrafast laser parameters are required. The generated nanogratings need to be measured using orthogonally polarized light sources. Once the nanograting is determined to be generated, the line will have polarization-dependent loss. Generally, polarization-dependent loss is less than insertion loss; to make the polarization-dependent loss greater than the insertion loss, specific laser parameters need to be set.

[0047] In this invention, the etched lines are aligned with the fiber core axis, and the etched lines can be arranged along the fiber core axis with fixed positions, thus avoiding the need to control the amount of etched line deviation and reducing the complexity of the etched lines.

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

[0049] Example 3

[0050] The passive mode-locked fiber laser based on a nanograting polarizer described in Example 2 differs in that:

[0051] according to Figure 4 The laser system was constructed using an optical path diagram. The length of the YDF gain fiber used in the experiment was 0.33m, the spacing length of the nanograting-based polarizer was 25cm, and the remaining fibers were single-mode fibers (HI1060, CORNING). The total length of the ring cavity was 8.3m. In the experiment, the polarization correlation loss of the nanograting-based polarizer was 3.2dB, and the insertion loss was less than 10dB. To achieve mode-locking, a pump light with a wavelength of 976nm and an average power of 783mW was input. By adjusting the two polarizer circuits (PCs), stable mode-locking of the laser system was achieved. The mode-locking results are as follows: Figure 5 This is a schematic diagram of the pulse sequence displayed on the oscilloscope in mode-locked mode; Figure 6 This is a schematic diagram of the FFT spectrum displayed on the oscilloscope in mode-locked mode; Figure 5 In the diagram, the horizontal axis represents time, and the vertical axis represents the normalized voltage value. Figure 5 In the image, the oscilloscope detects a sequence of mode-locked pulse signals with an interval of 40 ns between adjacent pulse signals. Figure 6 In the diagram, the horizontal axis represents the repetition frequency, and the vertical axis represents the intensity value. Figure 6 In the test, after the oscilloscope pulse sequence underwent FFT transformation, the fundamental frequency was 24.09MHz, the measured resolution bandwidth was 1.46kHz, the range was 458kHz, the signal-to-noise ratio (SNR) between the fundamental frequency and adjacent frequencies was 49dB, and the SNR between the fundamental frequency and background noise was 51dB. The oscilloscopes used were ROHDE & SCHWARZ RTB2004 2.5GSa / s and 5GSa / s.

[0052] Figure 7 This is the spectrum displayed by the spectrometer in mode-locked mode; Figure 7 In the diagram, the horizontal axis represents wavelength, and the vertical axis represents intensity. Figure 7 The output light has a spectral center wavelength of 1032 nm and a 3 dB spectral bandwidth of 2.5 nm. The spectrometer used is a YOKOGAWA AQ6370D.

[0053] Figure 8 It is the pulse profile diagram displayed by the autocorrelation instrument in mode-locked state; Figure 8 In the diagram, the horizontal axis represents time, and the vertical axis represents the intensity value. Figure 8In the image, at a scan range of 150 ps, ​​the pulse appears as a sharp, straight line. When the scan range is 15 ps (inset), the fitted pulse width of the peak is 220 fs, which closely matches the typical characteristics of noise-like pulses. The autocorrelator used is an APPulseCheck 2.2.103.

[0054] Figure 9 This is a schematic diagram of the power stability curve measured by the power meter; Figure 9 In the diagram, the horizontal axis represents time, and the vertical axis represents output power. Figure 9 The output power was monitored for 3 hours, and the root mean square (RMS) of the output power was 0.53% over those 3 hours. As shown in the inset, the maximum power fluctuation over those 3 hours was 0.02 milliwatts. The power meter and probe used were Thorlabs PM100D&S145C models. Figure 10 This is a schematic diagram showing the relationship between the pump input power and the annular cavity output power measured by the power meter;

[0055] Figure 10 In the diagram, the horizontal axis represents pump power, and the vertical axis represents output power. Figure 10 In the experiment, experimental data from three points were selected and fitted using a straight line. The slope of the line was 0.00141, indicating that the loss within the annular cavity is significant and requires further optimization. The power meter and probe used were 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, center wavelength 1032 nm; Autocorrelation: noise-like pulse, pulse width could not be measured at a scan range of 150 ps; pulse width of 220 fs was measured at a scan range of 15 ps; Power stability: RMS was 0.53% after measurement time > 3 h; Relationship between pump power and output power: three points were selected, and the slope of the fitted straight line was 0.001407.

[0057] Example 4

[0058] The operating method of the passive mode-locked fiber laser described in any of Examples 1-3 includes:

[0059] First, the pump light from the 976nm pump source is passed through a wavelength division multiplexer and enters the gain fiber. The gain fiber absorbs the pump light and generates spontaneous emission light, which includes a 1032nm signal light. Due to the unidirectional isolation function of the polarization-independent isolator, the generated spontaneous emission light can only be emitted in a polarization-independent manner. Figure 4The optical path is cyclically amplified in a clockwise direction. Due to the balance between loss and gain in the ring cavity, only the 1032nm signal light is amplified in the ring cavity. At this time, the polarization controller (PC2) controls the polarization state of the signal light. By controlling the polarization controller (PC2), the polarization state of the signal light is made into an elliptic polarization state.

[0060] Then, the polarization controller (PC1) is controlled to ensure that the central portion of the signal light pulse has the highest transmittance when passing through the nanograting lines. This transmittance is then continuously circulated through the ring cavity, ultimately yielding... Figure 7 The noise pulse mode-locking shown;

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

Claims

1. A passively mode-locked fiber laser based on nanograting-based polarizer, characterized in that, It comprises: a pump source, a wavelength division multiplexer, two polarization controllers, two nanometer grating lines, a single-mode optical coupler, a polarization-independent isolator and a gain fiber; The two nanometer grating lines are arranged between the two polarization controllers; 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 nanometer grating lines have the equivalent effect of a bulk polarizer and the two polarization controllers together 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 that the resonant light runs in one direction in the ring cavity; and the gain fiber converts the pump light 976nm into resonant light 1032m. The length of the two nanometer grating lines is 1-2mm, and the spacing length between the two nanometer grating lines is 1-25cm. The method for obtaining the two nanometer grating lines comprises: using ultrafast laser to directly write at the center position of the fiber core, and the polarization direction of the ultrafast laser is perpendicular to the fiber core axis, and the two nanometer grating lines are directly written.

2. The nanograting-based polarizer-locked mode-locked fiber laser of claim 1, wherein, The length of the two nanometer grating lines is 1mm, and the spacing length between the two nanometer grating lines is 25cm.

3. The nanograting-based polarizer-locked mode-locked fiber laser of claim 1, wherein, The length of the gain fiber is 0.33-1m, and the total length of the ring cavity is 5-40m.

4. The nanograting-based polarizer-locked mode-locked fiber laser of claim 1, wherein, The length of the gain fiber is 0.33m, and the total length of the ring cavity is 8.3m.

5. A method of operating a passively mode-locked fiber laser as claimed in any of claims 1-4, characterized in that, It comprises: Firstly, the pump source outputs pump light at 976nm, which enters the gain fiber through the wavelength division multiplexer. After the gain fiber absorbs the pump light, it generates spontaneous emission light, which contains signal light at 1032nm. Only the signal light at 1032nm 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 becomes elliptical polarization state; Then, the polarization controller is controlled to make the center part of the signal light pulse have the highest transmittance when passing through the nanometer grating line. At this time, the continuous circulation in the ring cavity obtains a noise-like pulse mode-locked; Finally, the single-mode fiber coupler outputs 10% of the energy in the ring cavity.

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