Deep blue random fiber laser based on femtosecond active phase-shifted fiber grating

Abstract

The application relates to a deep blue random fiber laser based on a femtosecond active phase shift fiber grating and relates to the technical field of lasers. The application comprises a 900nm random laser seed light source, a collimator, a first plano-convex lens, a frequency doubling crystal, a temperature control platform, a second plano-convex lens and a beam splitter prism; the 900nm random laser enters the collimator; the light beam is incident into the frequency doubling crystal, the frequency doubling crystal is high-efficiency absorbed to the 900nm waveband random laser through a CuFe2O4 metal nanoparticle film, and the output end face is coated with a 450nm antireflection film; the first plano-convex lens and the second plano-convex lens make the optimal focusing waist spot of the 900nm random laser be at the center position of the frequency doubling crystal; the 900nm and 450nm random lasers are collimated through the second plano-convex lens, the 900nm random laser is separated through the beam splitter prism, and the 450nm deep blue random laser output is obtained. The application obtains the blue random laser output with high stability, low phase noise and low coherence.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of laser technology, in particular to a deep blue random fiber laser based on femtosecond active phase-shift fiber grating. BACKGROUND

[0002] Random fiber lasers have rapidly developed in applied and basic science fields due to their low lasing threshold, high output power, high conversion efficiency, low phase noise, good output directivity and simple structure. In recent years, with the rapid development of random fiber laser technology, special wavebands such as visible light random fiber lasers have entered the public view. Blue random fiber lasers have the application value of traditional blue lasers in underwater communication, laser display and biomedical fields, and also have the characteristics of low coherence and speckle-free imaging of random fiber lasers. The effective combination of the two can provide strong support for high-quality laser imaging, and the application prospect is very bright. Now green random fiber lasers have realized random laser output, but most of the blue lasers currently remain in traditional single-frequency blue fiber lasers and semiconductor blue lasers, but their stability is poor, frequency noise is large, and conversion efficiency is low, which still restricts the further development and application of blue fiber lasers. Especially the research progress of deep blue fiber lasers with a waveband range of 450 nm is extremely slow, and deep blue random fiber lasers are rarely researched, which seriously hinders the development of underwater detection, laser imaging, high-precision welding and biomedical fields. Therefore, the design and construction of 450 nm deep blue random fiber lasers with high stability, low noise and high energy density are urgently needed to be solved.

[0003] Femtosecond laser micro-nano processing technology has become the main technical means for the preparation of phase-shift fiber Bragg gratings due to its characteristics of no thermal effect, high mechanical strength and flexible control. The phase-shift fiber Bragg grating is a special fiber Bragg grating, which introduces a half-period mutation in the continuous grating refractive index modulation. The phase-shift fiber Bragg grating is directly inscribed on the Nd-doped 3+ Direct inscription of phase-shift fiber Bragg grating (PS-FBG) on Nd-doped silica gain fiber can eliminate the direct fusion loss of traditional method between fiber grating and gain fiber, and use the narrow linewidth characteristics of phase-shift grating to generate blue random laser output with high spectral purity. Since the reflection spectrum of PS-FBG is very narrow, and the reflection characteristics are relatively low sensitive to environmental factors such as temperature and stress, the fiber laser based on PS-FBG has excellent stability, which is particularly important for long-term operation and complex environment applications. The effective combination of Nd-doped 3+ gain fiber and frequency selection device PS-FBG makes the blue random fiber laser based on PS-FBG more easily integrated with other optical devices, forming a multifunctional blue random fiber laser prototype system.

[0004] Nonlinear frequency conversion technology can effectively convert laser wavelength to different frequency bands by means of nonlinear medium, greatly widening the output wavelength range of laser and opening up a new path for the diversified output of blue random fiber laser. With the leap of crystal material preparation technology, high-performance periodically poled nonlinear crystals such as periodically poled lithium niobate (PPLN), lithium tantalate (PPLT) and potassium titanium oxide phosphate (PPKTP) have emerged as the times require, all of which can realize efficient quasi-phase matching frequency doubling. Among them, PPLN crystal has become the focus of the field due to its excellent nonlinear optical properties, high damage resistance, low matching temperature requirement and cost advantage. However, the absorption of PPLN crystal in the short wavelength region (such as 450 nm) will increase significantly, resulting in a large loss of frequency-doubled light. In addition, under high-power pumping, PPLN crystal will produce significant thermal effects due to the absorption of pump light and frequency-doubled light, resulting in uneven temperature distribution inside the crystal, which further affects the phase matching condition and nonlinear conversion efficiency. Therefore, it is necessary to optimize the design of PPLN crystal to improve the purity, loss threshold and frequency doubling efficiency of the crystal. First, Zn ions are doped into PPLN crystal to prepare Zn:LiNbO3 crystal, in which Zn ions can occupy Li or Nb sites in LiNbO3, reducing photorefraction effect through charge compensation and defect structure change. Because Zn ions can stabilize the crystal lattice structure and reduce defects induced by light, Zn:LiNbO3 crystal can also improve its own light damage resistance, ultimately improving the damage threshold of the frequency doubling crystal. To reduce the loss of fundamental light and frequency-doubled light caused by end face reflection of the crystal, a solution containing bimetallic oxide nanoparticles (CuFe2O4) is uniformly deposited on the input end face of the Zn:LiNbO3 crystal by chemical deposition method. By precisely controlling and designing the shape, size and arrangement of CuFe2O4 metal nanoparticles, the Localized Surface Plasmon Resonance (LSPR) peak is adjusted to near 900 nm. These plasmonic effects can localize the optical field in nanoscale, thereby greatly enhancing the absorption of 900 nm band random laser by PPLN crystal. Then a 450 nm antireflection coating is deposited on the output end face of the PPLN crystal to enhance the output of the frequency-doubled light. The entire coated Zn:LiNbO3 frequency doubling crystal is fixed on a temperature control platform by a clamp, and the temperature control platform uses a semiconductor cooler (TEC) with a temperature control accuracy of ±0.01℃. By adjusting the temperature control device to the optimal quasi-phase matching temperature, the fundamental light beam is doubled in the crystal to realize 450 nm random fiber laser output.

[0005] In summary, the deep blue random fiber laser has not been reported in detail so far, and its output characteristics have been less studied. Therefore, it is urgent to design and build a deep blue random fiber laser to promote the development of underwater communication, laser imaging and biomedical fields. SUMMARY

[0006] The application aims to design a deep blue random fiber laser based on femtosecond active phase-shift fiber grating, combine Zn:LiNbO3 crystal coated with CuFe2O4 metal nanoparticles on the end face as a frequency doubling structure, and adopt a new method of spatial single-pass frequency doubling to obtain deep blue random laser, which can obtain high stability, low phase noise and low coherence of blue random laser output.

[0007] To achieve the above purpose, the application provides a deep blue random fiber laser based on femtosecond active phase-shift fiber grating, which comprises a 900nm random laser seed light source, a collimator, a first plano-convex lens, a frequency doubling crystal, a temperature control platform, a second plano-convex lens and a beam splitter prism; the 900nm random laser generated by the 900nm random laser seed light source enters the collimator for collimation to reduce the loss of the seed light; the collimated light beam is focused by the first plano-convex lens and then incident into the frequency doubling crystal coated with Zn:LiNbO3, the CuFe2O4 metal nanoparticle film on the input end surface of the frequency doubling crystal enables the frequency doubling crystal to efficiently absorb the 900nm band random laser, and the output end surface is coated with a 450nm antireflection film to reduce the loss of the frequency doubled light; the first plano-convex lens and the second plano-convex lens enable the optimal focusing waist of the 900nm random laser to be at the center position of the frequency doubling crystal, so as to maximize the frequency doubling conversion efficiency; the temperature control platform is arranged at the bottom of the frequency doubling crystal, the temperature is adjusted to the optimal quasi-phase matching temperature, then the 900nm and 450nm random lasers pass through the second plano-convex lens to collimate the light beam, and then the 900nm random laser is separated by the beam splitter prism, and finally the 450nm deep blue random laser output is obtained.

[0008] Further, the 900nm random laser seed light source comprises a semiconductor laser, a wavelength division multiplexer, a Nd-doped gain fiber, an active phase-shift fiber Bragg grating, a long-distance single-mode fiber, a filter and a coupler; 808nm pump light is injected into the Nd-doped gain fiber through the wavelength division multiplexer, gain amplification is obtained by the pump light through population inversion, and the gain-amplified light propagates into the active phase-shift fiber Bragg grating, the light with a center wavelength of 900nm is reflected, then propagates counterclockwise through the coupler and the filter into the long-distance single-mode fiber, a random distributed feedback is provided by the inherent backscattering of the long-distance single-mode fiber, most of the backscattered light propagates clockwise through the wavelength division multiplexer again into the Nd-doped gain fiber for amplification, and part of the forward-scattered light also propagates into the active phase-shift fiber Bragg grating for reflection. 3+ The gain-amplified light propagates into the active phase-shift fiber Bragg grating, the light with a center wavelength of 900nm is reflected, then propagates counterclockwise through the coupler and the filter into the long-distance single-mode fiber, a random distributed feedback is provided by the inherent backscattering of the long-distance single-mode fiber, most of the backscattered light propagates clockwise through the wavelength division multiplexer again into the Nd-doped gain fiber for amplification, and part of the forward-scattered light also propagates into the active phase-shift fiber Bragg grating for reflection. 3+ The gain-amplified light propagates into the active phase-shift fiber Bragg grating, the light with a center wavelength of 900nm is reflected, then propagates counterclockwise through the coupler and the filter into the long-distance single-mode fiber, a random distributed feedback is provided by the inherent backscattering of the long-distance single-mode fiber, most of the backscattered light propagates clockwise through the wavelength division multiplexer again into the Nd-doped gain fiber for amplification, and part of the forward-scattered light also propagates into the active phase-shift fiber Bragg grating for reflection. 3+ The gain-amplified light propagates into the active phase-shift fiber Bragg grating, the light with a center wavelength of 900nm is reflected, then propagates counterclockwise through the coupler and the filter into the long-distance single-mode fiber, a random distributed feedback is provided by the inherent backscattering of the long-distance single-mode fiber, most of the backscattered light propagates clockwise through the wavelength division multiplexer again into the Nd-doped gain fiber for amplification, and part of the forward-scattered light also propagates into the active phase-shift fiber Bragg grating for reflection.

[0009] Further, the 900nm random laser seed light source further comprises a heat dissipation platform, the Nd-doped 3+ The gain optical fiber is placed on the heat dissipation platform to prevent the active phase-shifted fiber Bragg grating from being affected by temperature.

[0010] Further, the writing structure of the active phase-shifted fiber Bragg grating comprises a broadband light source, a spectrometer, a connector, a circulator, a femtosecond laser light source, an aperture, a camera, a dichroic mirror, an oil lens, the Nd-doped 3+ optical fiber, and a phase-shifted fiber Bragg grating; the femtosecond laser beam emitted by the femtosecond laser light source is divided into two beams by the dichroic mirror after passing through the aperture, one beam of light enters the camera for real-time observation of the grating preparation form, and the other beam of light is focused on the Nd-doped 3+ optical fiber by the oil lens for laser writing; the broadband light source is connected to the circulator through the connector, the incident light emitted by the broadband light source enters the phase-shifted fiber Bragg grating through the circulator, and the light with the center wavelength is reflected to the spectrometer for real-time monitoring of the spectrum change.

[0011] Further, the phase-shifted fiber Bragg grating is internally provided with a phase interval.

[0012] Further, the active phase-shifted fiber Bragg grating further comprises a displacement platform, and the Nd-doped 3+ optical fiber is placed on the displacement platform.

[0013] Further, the active phase-shifted fiber Bragg grating further comprises a fixed platform, and the oil lens is installed on the fixed platform.

[0014] Further, the preparation method of the deep blue random fiber laser comprises the following steps:

[0015] Step S1, a high-reflection phase-shifted fiber Bragg grating is prepared on the active Nd-doped 3+ gain optical fiber by using the femtosecond laser line-by-line writing method, so as to obtain an active phase-shifted fiber Bragg grating with a center wavelength of 900nm, a reflectivity greater than 90%, and a 3dB bandwidth less than 0.1nm;

[0016] Step S2, connecting the wavelength division multiplexer input end to the 808nm pump light source;

[0017] Step S3, sequentially connecting the Nd-doped 3+ gain optical fiber, the long-distance single-mode optical fiber, the filter, and the coupler to the output end of the wavelength division multiplexer;

[0018] Step S4, connecting the coupler 90% output end to the wavelength division multiplexer back end port for repeated use of the pump;

[0019] Step S5, the pump power of the 808nm semiconductor laser is increased, when the gain of the random laser system is greater than the loss, the 900nm random laser output is obtained at the 10% output port of the coupler, and the spectral performance is tested and analyzed;

[0020] Step S6, the 900nm random laser is input into the spatial single-pass frequency doubling system through the collimator, then the beam is collimated through the collimating mirror, the 900nm random laser is transmitted to the frequency doubling crystal of the metal nanoparticle film Zn:LiNbO3 through the first plano-convex lens to realize frequency doubling, then the 900nm fundamental frequency light and the 450nm frequency-doubled light beam are transmitted to the second plano-convex lens through the output end of the frequency doubling crystal to re-collimate the output light beam of the frequency doubling crystal; finally, the 900nm random laser and the 450nm deep blue random laser are separated through the beam splitter prism, and the deep blue random laser is tested and analyzed.

[0021] Further, the writing step of the active phase-shifted fiber Bragg grating is as follows:

[0022] Step S11: the Nd-doped 3+ The optical fiber is placed on the holder of the three-dimensional high-precision electrically-controlled writing platform and a pre-tightening force is applied;

[0023] Step S12: sequentially turn on the seed light source and the amplification stage to obtain the femtosecond laser output;

[0024] Step S13: adjust the femtosecond laser output power and the focused spot position;

[0025] Step S14: design the period, length and focused energy of the active phase-shifted fiber Bragg grating through the computer software;

[0026] Step S15: connect one end of the active phase-shifted fiber Bragg grating to the circulator, connect the other two ends to the optical spectrum analyzer and the broadband light source respectively, then set the moving speed of the displacement platform and the scanning length of the laser, and write the optical fiber, while observing the reflection spectrum of the active phase-shifted fiber Bragg grating in the optical spectrum analyzer.

[0027] The beneficial effects of the present application are:

[0028] 1. A kind of active Nd-doped 3+ The high-reflection phase-shifted fiber Bragg grating is used as a device integrating gain and frequency selection of the random fiber laser, so that the structure of the laser is more compact and the fusion loss between optical fibers is reduced.

[0029] 2. The surface of the Zn:LiNbO3 frequency doubling crystal is plated with a CuFe2O4 metal nanoparticle film by chemical deposition method, so as to increase the absorption rate of the frequency doubling crystal to 900nm band random laser.

[0030] 3、The application realizes 450nm deep blue random fiber laser output by designing and preparing active fiber grating and coating film frequency doubling crystal and other key structures, and building random fiber laser based on space single pass frequency doubling structure, and the optimization design of frequency selection device and frequency doubling crystal reduces the transmission loss of the laser system and improves the output characteristics of the blue random laser. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 It is a schematic diagram of femtosecond laser writing active high-reflection phase-shift fiber Bragg grating structure of the application;

[0032] Figure 2 It is a schematic diagram of a ring cavity doped with Nd 3+ It is a schematic diagram of a random fiber laser seed source system;

[0033] Figure 3 It is a schematic diagram of a 450nm deep blue random fiber laser system of the application;

[0034] Figure 4 It is a transmission spectrum of a 900nm phase-shift fiber grating of the application;

[0035] Figure 5 It is a 900nm random laser diagram of the application;

[0036] Figure 6 It is a 450nm random laser diagram of the application.

[0037] In the figure: 1-broadband light source; 2-spectrometer; 3-connector; 4-coupler; 5-femtosecond laser light source; 6-diaphragm; 7-camera; 8-dichroic mirror; 9-fixing platform; 10-oil lens; 11-doped Nd 3+ fiber; 12-displacement platform; 13-grating internal phase interval; 14-phase-shift fiber Bragg grating; 15-semiconductor laser; 16-wave division multiplexer; 17-doped Nd 3+ gain fiber; 18-active phase-shift fiber Bragg grating; 19-heat dissipation platform; 20-long distance single mode optical fiber; 21-filter; 22-coupler; 24-collimator; 25-first plano-convex lens; 26-frequency doubling crystal; 27-temperature control platform; 28-second plano-convex lens; 29-splitting prism. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the application will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0039] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above drawings are used to distinguish similar objects, and do not necessarily have to describe a specific order or sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or units does not have to be limited to those steps or units clearly listed, but can include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0040] In the present application, the terms "upper", "lower", "left", "right", "front", "back", "top", "bottom", "inner", "outer", "middle", "vertical", "horizontal", "lateral", "longitudinal" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings. These terms are mainly used to better describe the present application and its embodiments, and are not intended to limit the indicated devices, elements or components to have a specific orientation, or to be constructed and operated in a specific orientation.

[0041] In addition, in addition to indicating the orientation or positional relationship, the above-mentioned partial terms can also be used to indicate other meanings, for example, the term "upper" can also be used to indicate a certain dependent relationship or connection relationship in some cases. For those skilled in the art, the specific meaning of these terms in the present application can be understood according to the specific circumstances.

[0042] In addition, the terms "mount", "set", "provided with", "connected", "connected", "sleeved" should be broadly understood. For example, it can be a fixed connection, a detachable connection, or a monolithic structure; it can be a mechanical connection, or an electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, or internal communication between two devices, elements or components. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0043] The present application prepares a phase-shift fiber Bragg grating with a center wavelength of 900nm on an active Nd-doped gain fiber by femtosecond laser processing technology, and constructs a resonant cavity with the inherent back Rayleigh scattering in a 20km single-mode optical fiber to emit random laser. The active Nd-doped gain fiber can also provide gain for the laser system, and when the gain of the random laser system is greater than the loss, 900nm random laser output can be obtained. The Nd-doped gain fiber can also provide gain for the laser system, and when the gain of the random laser system is greater than the loss, 900nm random laser output can be obtained. The Nd-doped gain fiber can also provide gain for the laser system, and when the gain of the random laser system is greater than the loss, 900nm random laser output can be obtained. 3+ The gain fiber can also provide gain for the laser system, and when the gain of the random laser system is greater than the loss, 900nm random laser output can be obtained. The Nd-doped gain fiber can also provide gain for the laser system, and when the gain of the random laser system is greater than the loss, 900nm random laser output can be obtained. 3+ The gain fiber can also provide gain for the laser system, and when the gain of the random laser system is greater than the loss, 900nm random laser output can be obtained. The Nd-doped gain fiber can also provide gain for the laser system, and when the gain of the random laser system is greater than the loss, 900nm random laser output can be obtained. 3+The random fiber laser is used as a random laser light source. A Zn:LiNbO3 crystal with a CuFe2O4 metal nanoparticle end face is used as a frequency doubling structure to improve the light absorption rate and damage threshold of the frequency doubling crystal to the 900 nm waveband. The optimal quasi-phase matching temperature is achieved by adjusting the temperature control platform, so that the fundamental frequency light beam is doubled in the crystal, and the 900 nm random laser and the 450 nm positive random blue light generated by frequency doubling are output from the rear end of the crystal. The output light beam is collimated by a plano-convex lens, which is convenient for subsequent beam testing and analysis. Finally, the 450 nm frequency-doubled light and the 900 nm fundamental frequency light are separated by a beam splitter prism, and the 450 nm deep blue random laser output is achieved, which provides a new type of light source for laser projection, laser imaging, laser radar, laser display and other fields.

[0044] Referring to Figure 1 The embodiment discloses a deep blue random fiber laser based on a femtosecond active phase-shift fiber grating, which comprises a 900 nm random laser seed light source, a collimator 24, a first plano-convex lens 25, a frequency doubling crystal 26, a temperature control platform 27, a second plano-convex lens 28 and a beam splitter prism 29. The 900 nm random laser generated by the 900 nm random laser seed light source enters the collimator 24 for collimation to reduce the seed light loss. The collimated light beam is focused by the first plano-convex lens 25 and enters the frequency doubling crystal 26 coated with Zn:LiNbO3. The CuFe2O4 metal nanoparticle film on the input end surface makes the frequency doubling crystal 26 efficiently absorb the 900 nm waveband random laser, and the output end surface is coated with a 450 nm antireflection film to reduce the frequency-doubled light loss. The first plano-convex lens 25 and the second plano-convex lens 28 make the optimal focusing waist of the 900 nm random laser at the center position of the frequency doubling crystal 26 to maximize the frequency doubling conversion efficiency. The temperature control platform 27 is arranged at the bottom of the frequency doubling crystal 26, and the temperature is adjusted to the optimal quasi-phase matching temperature. Then, the 900 nm and 450 nm random lasers pass through the second plano-convex lens 28 for collimation, and then pass through the beam splitter prism 29 to separate the 900 nm random laser. Finally, the 450 nm deep blue random laser output is obtained.

[0045] Further optimization of the technical scheme, the 900 nm random laser seed light source comprises: a semiconductor laser 15, a wavelength division multiplexer 16, a Nd 3+ doped gain fiber 17, an active phase-shift fiber Bragg grating 18, a long-distance single-mode optical fiber 20, a filter 21 and a coupler 22. The 808 nm pump light is injected into the Nd 3+In the gain fiber 17, the gain is amplified by the population inversion pump light and propagates into the active phase-shifted fiber Bragg grating 18, the light with the center wavelength of 900 nm is reflected, then propagates into the long-distance single-mode fiber 20 through the coupler 22 and the filter 21 in a counterclockwise direction, due to the inherent back Rayleigh scattering providing random distributed feedback, most of the back Rayleigh scattering reflected light will propagate into the Nd-doped 3+ The gain fiber 17 is amplified, and part of the forward Rayleigh scattering light also propagates to the active phase-shifted fiber Bragg grating 18 for reflection, when the gain of the random laser system is greater than the loss, the 900 nm random laser is obtained at the tail end of the coupler 22.

[0046] Further optimize the technical scheme, the 900 nm random laser seed light source further includes a heat dissipation platform 19, the Nd-doped 3+ The gain fiber 17 will be placed on the heat dissipation platform 19 to prevent the active phase-shifted fiber Bragg grating 18 from being affected by temperature.

[0047] Further optimize the technical scheme, the writing structure of the active phase-shifted fiber Bragg grating 18 includes a broadband light source 1, a spectrometer 2, a connector 3, a circulator 4, a femtosecond laser light source 5, an aperture 6, a camera 7, a dichroic mirror 8, an oil lens 10, an Nd-doped 3+ The fiber 11, the phase-shifted fiber Bragg grating 14; the femtosecond laser beam emitted by the femtosecond laser light source 5 is divided into two beams by the dichroic mirror 8 after passing through the aperture 6, one beam enters the camera 7 for real-time observation of the grating preparation form, and the other beam is focused on the Nd 3+ The laser writing is performed on the fiber 11; the broadband light source 1 is connected to the circulator 4 through the connector 3, the incident light emitted by the broadband light source 1 enters the phase-shifted fiber Bragg grating 14 through the circulator 4, and the light with the center wavelength is reflected to the spectrometer 2 for real-time monitoring of the spectral change.

[0048] Further optimize the technical scheme, the phase-shifted fiber Bragg grating 14 is internally provided with a phase interval 13.

[0049] Further optimize the technical scheme, the active phase-shifted fiber Bragg grating 18 further includes a displacement platform 12, and the Nd-doped 3+ The fiber 11 is placed on the displacement platform 12.

[0050] Further optimize the technical scheme, the active phase-shifted fiber Bragg grating 18 further includes a fixed platform 9, and the oil lens 10 is installed on the fixed platform 9.

[0051] Further optimize the technical scheme, the preparation method of the deep blue random fiber laser includes the following steps:

[0052] Step S1: Active Nd-doped materials are fabricated using a femtosecond laser line-by-line writing method. 3+ A high anti-phase-shift fiber Bragg grating was fabricated on a gain fiber to obtain an active phase-shift fiber Bragg grating with a reflectivity greater than 90%, a 3dB bandwidth of less than 0.1nm, and a center wavelength of 900nm.

[0053] Step S2: Connect the 808nm pump light source to the input of the wavelength division multiplexer;

[0054] Step S3: Connect Nd-doped nanofibers etched with PS-FBG sequentially to the output of the wavelength division multiplexer. 3+ Gain fiber, long-distance single-mode fiber, filters and couplers;

[0055] Step S4: Connect the 90% output of the coupler to the back port of the wavelength division multiplexer for pump reuse.

[0056] Step S5: Increase the pump power of the 808nm semiconductor laser. When the gain of the random laser system is greater than the loss, the 10% output port of the coupler obtains a 900nm random laser output, and its spectral performance is tested and analyzed.

[0057] In step S6, the 900nm random laser is collimated by a collimator and input into the spatial single-pass frequency doubling system. Then, it is collimated by a collimating lens. The 900nm random laser propagates through a first plano-convex lens to a frequency doubling crystal with a metal nanoparticle film Zn:LiNbO3 to achieve frequency doubling. Then, the 900nm fundamental frequency light and the 450nm frequency doubling beam propagate through the output end of the frequency doubling crystal to a second plano-convex lens to re-collimate the output beam of the frequency doubling crystal. Finally, the laser is separated from the 450nm deep blue random laser by a beam splitter, and the deep blue random laser is tested and analyzed.

[0058] Nd doping 3+ The random fiber laser unit has a 5m gain fiber, an active phase-shifting fiber Bragg grating with a reflectivity exceeding 90%, a 3dB bandwidth of less than 0.1nm, and a grating length of 3mm. A 20km single-mode fiber's inherent Rayleigh scattering and the phase-shifting fiber Bragg grating form a resonant cavity. An 808nm semiconductor laser (LD) is injected into the ring cavity via a fiber wavelength division multiplexer (WDM) for pumping. After repetitive gain amplification and feedback oscillation, a 900nm random laser is finally generated and output from the 10% port of the coupler. A heat dissipation platform effectively scatters the gain fiber to prevent the active PS-FBG from being affected by temperature.

[0059] The technical solution has been further optimized, and the steps for writing active phase-shifting fiber Bragg gratings are as follows:

[0060] Step S11: Add Nd-doped 3+The optical fiber is placed on the holder of the three-dimensional high-precision electronically controlled writing platform and a preload is applied;

[0061] Step S12: Sequentially turn on the seed light source and amplification stage to obtain femtosecond laser output;

[0062] Step S13: Adjust the femtosecond laser output power and focused spot position;

[0063] Step S14: Design the parameters of the active phase-shifting fiber Bragg grating, including period, line length, and focusing energy, using computer software;

[0064] Step S15: Connect the active phase-shifting fiber Bragg grating to one end of the circulator, and connect the other two ends to the spectrometer and the broadband light source respectively. Then set the moving speed of the displacement platform and the laser scanning length and write the fiber. At the same time, observe the reflection spectrum of the active phase-shifting fiber Bragg grating in the spectrometer.

[0065] This invention provides a 450nm deep blue random fiber laser based on a femtosecond phase-shifted fiber Bragg grating (PS-FBG). Utilizing the flexibility, high output power, and temperature resistance of femtosecond laser technology, a neodymium-doped (Nd) random fiber laser is fabricated using a line-by-line etching method. 3+ A high-reflectivity phase-shifting fiber Bragg grating was fabricated in a quartz gain fiber as a gain-selective device for a blue light random fiber system. Among these, Nd-doped fiber... 3+ Optical fibers provide gain for the laser system, and fiber optic gratings serve as frequency-selective devices for the random laser system. The efficient combination of gain fiber and frequency-selective device reduces splice loss between fibers, and the narrow linewidth characteristic of phase-shift gratings enables high-spectral-purity blue random laser output. The inherent Rayleigh scattering in a 20km long-distance single-mode fiber provides optical feedback for random laser generation. The phase-shift fiber grating and backscattering in the long-distance single-mode fiber form a so-called resonant cavity, thus obtaining a 900nm single-wavelength random laser output. A frequency doubling system is built based on a surface-coated periodically polarized lithium niobate crystal module, and a 450nm deep blue random laser output is achieved using an external cavity single-pass frequency doubling method. This invention utilizes femtosecond lasers with active gain Nd-doped lasers. 3+ Inscribing phase-shifted fiber gratings onto optical fibers enables integrated gain feedback in random laser systems, resulting in a more compact and efficient laser structure. Based on the inherent Rayleigh scattering of single-mode fibers as optical feedback, and combined with the strong frequency selectivity of phase-shifted fiber gratings, a 450nm deep blue random laser output is achieved through an external frequency doubling structure. This laser can be widely used in speckle-free imaging, night vision illumination, underwater communication, and biomedicine.

[0066] Preparing active Nd-doped materials using femtosecond laser processing technology 3+High reverse phase shift fiber Bragg grating as a random laser system frequency selection device, the gain fiber and the frequency selection device are effectively combined to reduce the fusion transmission loss. It is beneficial to long distance single mode fiber inherent Rayleigh scattering to provide random distribution feedback and build Nd 3+ Random fiber laser system, realizing 900nm random laser output, reference Figure 4 With Figure 5 The semiconductor optical amplifier, plano-convex lens, coated Zn: LiNbO3 frequency doubling crystal, temperature control platform and beam splitter prism are used to build a random laser space single pass frequency doubling structure, realizing 450nm deep blue random laser output, reference Figure 6 The invented deep blue random fiber laser benefits from the low insertion loss and strong frequency selection characteristics of the active PS-FBG, and the high-efficiency absorption of 900nm fundamental light by the CuFe2O4 metal nanoparticle film based on LSPR effect, so that the 450nm deep blue random laser has high stability, low frequency jitter and relative intensity noise. Based on the low coherence and speckle-free imaging advantages of the random fiber laser, therefore, the invention of the 450nm blue random laser can effectively promote the application in the fields of laser display and laser imaging.

[0067] The above is only the preferred embodiment of the present application, and does not limit the technical scope of the present application in any way. Any slight modification, equivalent change and modification made according to the technical essence of the present application to the above embodiments are still within the scope of the technical solution of the present application.

Claims

1. A deep blue random fiber laser based on femtosecond active phase-shifted fiber grating, characterized in that, It comprises: 900nm random laser seed light source, collimator (24), first plano-convex lens (25), frequency doubling crystal (26), temperature control platform (27), second plano-convex lens (28) and beam splitter prism (29); the 900nm random laser generated by the 900nm random laser seed light source enters the collimator (24) for collimation to reduce the loss of the seed light; the collimated light beam is focused by the first plano-convex lens (25) and is incident into the frequency doubling crystal (26) coated with Zn:LiNbO3, the frequency doubling crystal (26) is efficiently absorbed by the CuFe2O4 metal nanoparticle film on the input surface, and the output surface is coated with a 450nm antireflection film to reduce the loss of the frequency doubled light; the first plano-convex lens (25) and the second plano-convex lens (28) make the best focusing waist of the 900nm random laser at the center position of the frequency doubling crystal (26), so as to maximize the frequency doubling conversion efficiency; the temperature control platform (27) is arranged at the bottom of the frequency doubling crystal (26), the temperature is adjusted to the best quasi-phase matching temperature, then the 900nm and 450nm random lasers are collimated by the second plano-convex lens (28) first, and then the 900nm random laser is separated by the beam splitter prism (29), finally the 450nm deep blue light random laser output is obtained; The 900nm random laser seed source comprises a semiconductor laser (15), a wavelength division multiplexer (16), a Nd-doped 3+ gain fiber (17), an active phase-shifted fiber Bragg grating (18), a long-distance single-mode fiber (20), a filter (21) and a coupler (22); 808nm pump light is injected into the Nd-doped 3+ gain fiber (17) through the wavelength division multiplexer (16), and is amplified by population inversion pump light and propagates into the active phase-shifted fiber Bragg grating (18), light with a center wavelength of 900nm is reflected, and then propagates counterclockwise through the coupler (22) and the filter (21) into the long-distance single-mode fiber (20), because of the inherent back Rayleigh scattering, most of the back Rayleigh scattering reflected light will again propagate clockwise through the wavelength division multiplexer (16) into the Nd-doped 3+ gain fiber (17) for amplification, part of the forward Rayleigh scattering light also propagates to the active phase-shifted fiber Bragg grating (18) for reflection, and when the gain of the random laser system is greater than the loss, the 900nm random laser is obtained at the tail end of the coupler (22).

2. The deep blue random fiber laser based on femtosecond active phase-shifted fiber grating according to claim 1, wherein, The 900 nm random laser seed source further comprises a heat dissipation platform (19), the Nd-doped 3+ The gain fiber (17) will be placed on the heat dissipation platform (19) to prevent the active phase-shifted fiber Bragg grating (18) from being affected by temperature.

3. The deep blue random fiber laser based on femtosecond active phase-shifted fiber grating according to claim 2, wherein, The inscription structure of the active phase-shifted fiber Bragg grating (18) comprises a broadband light source (1), a spectrometer (2), a connector (3), a circulator (4), a femtosecond laser light source (5), an aperture (6), a camera (7), a dichroic mirror (8), an oil lens (10), a Nd-doped 3+ fiber (11), a phase-shifted fiber Bragg grating (14); the femtosecond laser beam emitted by the femtosecond laser light source (5) is divided into two beams by the dichroic mirror (8) after passing through the aperture (6), one beam enters the camera (7) for real-time observation of the grating preparation morphology, and the other beam is focused on the Nd-doped 3+ fiber (11) by the oil lens (10) for laser inscription; the broadband light source (1) is connected to the circulator (4) through the connector (3), the incident light emitted by the broadband light source (1) enters the phase-shifted fiber Bragg grating (14) through the circulator (4), and the light conforming to the center wavelength is reflected to the spectrometer (2) for real-time monitoring of the spectral change.

4. The deep blue random fiber laser based on femtosecond active phase-shifted fiber grating according to claim 3, wherein, The phase shift fiber Bragg grating (14) is internally provided with a phase interval (13).

5. The deep blue random fiber laser based on femtosecond active phase-shifted fiber grating according to claim 3, wherein, The active phase-shifted fiber Bragg grating (18) further comprises a displacement stage (12), the Nd-doped 3+ The optical fiber (11) is placed on the displacement stage (12).

6. The deep blue random fiber laser based on femtosecond active phase-shifted fiber grating according to claim 3, wherein, The active phase shift fiber Bragg grating (18) further comprises a fixed platform (9), and the oil lens (10) is mounted on the fixed platform (9).

7. The deep blue random fiber laser based on femtosecond active phase-shifted fiber grating according to claim 5, wherein, The preparation method of the deep blue random fiber laser comprises the following steps: Step S1: Active Nd-doped materials are fabricated using a femtosecond laser line-by-line writing method. 3+ A high anti-phase-shift fiber Bragg grating was fabricated on a gain fiber to obtain an active phase-shift fiber Bragg grating with a reflectivity greater than 90%, a 3dB bandwidth of less than 0.1nm, and a center wavelength of 900nm. Step S2, connecting the 808nm pump light source to the input end of the wavelength division multiplexer; Step S3, connecting the Nd-doped fiber grating engraved with phase shift fiber grating at the output end of the wavelength division multiplexer in sequence 3+ Gain fiber, long distance single mode fiber, filter and coupler Step S4, connecting the 90% output end of the coupler to the back port of the wavelength division multiplexer for repeated use of the pump; Step S5, increasing the pump power of the 808nm semiconductor laser, when the gain of the random laser system is greater than the loss, the 10% output port of the coupler obtains the 900nm random laser output, and the spectral performance is tested and analyzed; Step S6, the 900nm random laser is collimated by the collimator and input into the spatial single-pass frequency doubling system, then the light beam is collimated by the collimating mirror, the 900nm random laser is transmitted to the frequency doubling crystal coated with the metal nanoparticle film Zn:LiNbO3 by the first plano-convex lens to realize frequency doubling, then the 900nm fundamental frequency light and the 450nm frequency doubled light beam are transmitted to the second plano-convex lens through the output end of the frequency doubling crystal to collimate the output light beam of the frequency doubling crystal; finally, the 900nm random laser and the 450nm deep blue light random laser are separated by the beam splitter prism, and the deep blue light random laser is tested and analyzed.

8. The deep blue random fiber laser based on femtosecond active phase-shifted fiber grating according to claim 7, wherein, The active phase shift fiber Bragg grating writing step is as follows: Step S11: Nd-doped 3+ The optical fiber is placed on the holder of the three-dimensional high-precision electrically-controlled writing platform and a pre-tightening force is applied. Step S12: sequentially turn on the seed light source and the amplifier stage to obtain the femtosecond laser output; Step S13: adjust the femtosecond laser output power and the focusing spot position; Step S14: design the parameters of the active phase shift fiber Bragg grating period, length and focusing energy through the computer software; Step S15: connecting one end of the active phase-shifted fiber Bragg grating with the circulator, connecting the other two ends with the optical spectrum analyzer and the broadband light source respectively, setting the moving speed of the displacement platform and the scanning length of the laser, and writing the fiber Bragg grating, while observing the reflection spectrum of the active phase-shifted fiber Bragg grating in the optical spectrum analyzer.

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