Dual-band mode-locked fiber laser based on polarization control of metasurface

Abstract

The invention discloses a dual-band mode-locked fiber laser based on metasurface polarization control, and belongs to the technical field of fiber lasers. Comprising a first loop, a second loop and a common end, the first loop and the second loop are connected with the gold cross-shaped metasurface through the common end, and the first loop and the second loop are independent of each other and do not affect each other. According to the dual-band ultrafast laser device, based on the metal surface plasmon effect, the gold cross-shaped metasurface mode locking device is prepared, gold cross-shaped metasurface structures which are different in length in the horizontal direction and the vertical direction are designed respectively, the polarization and nonlinear saturable absorption characteristics of the gold cross-shaped metasurface structures are utilized, and the dual-band ultrafast laser device is obtained. And meanwhile, mode-locked laser output in two wavebands is realized, and the lasers in the two wavebands are mutually independent and do not influence each other.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber lasers, and in particular to a dual-band mode-locked optical fiber laser based on polarization control of a metasurface. Background Art

[0002] Plasmonic metasurfaces are subwavelength two-dimensional surfaces with periodic structures that are artificially designed. Their nonlinear optical effects are determined by the equivalent polarizability of the primitives and the polarizability of the materials they constitute at the microscopic scale. The plasmon resonance phenomenon occurring on the metal surface can significantly enhance the absorption of light. This resonance phenomenon makes the thickness of the metasurface much smaller than the wavelength of the incident light, while the spatial configuration and orientation of its unit structure can finely control the amplitude and phase of the far-field light. These characteristics all follow the relevant principles of the generalized Snell-Descartes law. The goal of planar optics is to design ultrathin metasurfaces to simulate the performance of traditional large-volume optical components. It has been successfully applied to planar lenses, polarizers, holograms, and collimators. At the same time, the research of Moreau et al. shows that the absorption of light can be significantly enhanced on plasmonic metasurfaces. In this field, quasi-perfect absorbers have been widely studied for their potential applications in light filtering, photosensitive materials, and photovoltaics. However, it should be noted that in practical applications, few studies have focused on metasurfaces under pulsed laser irradiation, because in this case the optical nonlinear effect becomes very significant.

[0003] In the past two decades, the nonlinear absorption phenomenon of plasmonic devices has attracted widespread attention, especially because it can induce complex hot electron cascade reactions, which show unique application potential in many fields, such as photochemical reactions and ultrafast photodetection technology. Among them, the nonlinear absorption of gold plasmonic devices is particularly noteworthy because it can excite hot electron clouds on a sub-picosecond time scale, thereby triggering other physical or chemical processes. In this ultrafast electron excitation process, when the electron distribution becomes non-thermal, the states occupied by electrons below the Fermi level decrease, which is closely related to the change of transient absorption of the metal dielectric function. Studies have found that under strong light irradiation, the absorption of gold can reach saturation, which plays a key role in achieving pulse stabilization in laser systems. In fact, in order to generate stable ultrashort pulses and operate through mode locking, the laser cavity needs to have a saturable absorber with ultrafast relaxation time, nonlinear transfer function and high damage threshold. In order to achieve integration on fiber laser and waveguide laser platforms, researchers have developed a variety of saturable two-dimensional absorption materials in the past two decades. These include low-dimensional materials such as single-walled carbon nanotubes, graphene, topological insulators, and black phosphorus. Although these materials have nonlinear effects, they are usually limited to a few percentage points. In contrast, optical metasurfaces, as ultra-thin planar devices, are composed of metal or dielectric metafunctional primitives with spatial variations, which can locally control the light field on a subwavelength scale and enhance the interaction between light and matter. This enhancement makes the nonlinear optical effect more significant on plasmonic metasurfaces. On plasmonic metasurfaces, the light field is highly localized near the metal nanostructure, forming an extremely strong localized electromagnetic field. This localized electromagnetic field can greatly improve the efficiency of nonlinear optical processes, making nonlinear phenomena that originally require high-intensity light sources to observe easy to achieve on plasmonic metasurfaces, showing great potential. By combining with existing metal ions to form a new metasurface structure, and by precisely designing the geometric structure and material composition of the plasmonic metasurface, the polarization state, phase, and amplitude of light can be multi-dimensionally controlled on a subwavelength scale. It is worth noting that most previous studies used colloidal solutions of gold nanoparticles with dispersed sizes and random orientations as saturable absorbers, whose optical properties are closer to average properties and less relevant to the design of a single periodic structure. Therefore, developing new metasurface structures and utilizing their unique nonlinear optical properties is of great significance for promoting scientific and technological progress in related fields. Summary of the invention

[0004] The purpose of the present invention is to provide a dual-band mode-locked fiber laser based on polarization control of a metasurface. The dual-band ultrafast laser device using the metasurface as a saturable absorber can achieve mode-locked laser output that operates independently in different bands simultaneously.

[0005] To achieve the above object, the present invention provides a dual-band mode-locked fiber laser based on metasurface polarization control, comprising a first loop, a second loop and a common end, wherein the first loop and the second loop are both connected to the common end; The first loop includes a pump source 1, a wavelength division multiplexer 1, an erbium-doped optical fiber, a fiber isolator 1, a fiber coupler 1 and a polarization controller 1 arranged in a surrounding manner; The second loop includes a second pump source, a second wavelength division multiplexer, a thulium-doped optical fiber, a second optical fiber isolator, a second optical fiber coupler and a second polarization controller arranged in a surrounding manner; The common end includes a wavelength division multiplexer 3, a fiber collimator 1, a fiber collimator 2, a wavelength division multiplexer 4 and a golden cross metasurface which are arranged in sequence.

[0006] Pump source 1 and pump source 2 are used to provide light source for the laser; wavelength division multiplexer 1, wavelength division multiplexer 2, wavelength division multiplexer 3 and wavelength division multiplexer 4 are used to synthesize or separate different wavelengths; erbium-doped fiber and thulium-doped fiber are used to amplify optical signals; fiber isolator 1 and fiber isolator 2 are used to limit the direction of light so that the light can only be transmitted in one direction; polarization controller 1 and polarization controller 2 are used to control the polarization state of light and improve the output quality of the laser; fiber coupler 1 and fiber coupler 2 are used to realize the branching or combining of optical signals; fiber collimator 1 and fiber collimator 2 are used to convert the divergent light at the output end of the optical fiber into collimated light through the internal lens, and can also couple the parallel light in the free space into the optical fiber; golden cross metasurface is used as a saturable absorber mode-locked device to generate short pulse lasers.

[0007] Preferably, the golden cross metasurface adopts micro-nano processing technology, which specifically includes the following production steps: S1. Select a glass substrate with an indium tin oxide (ITO) coating as the processing material, and clean the sample in an ultrasonic wave with acetone, isopropyl alcohol solution, and deionized water to remove impurities; S2, drying the cleaned sample substrate with nitrogen, baking at 180° C. for 3 minutes, spin-coating a layer of positron anti-corrosion agent on the substrate, and baking again at 180° C. for 3 minutes; S3. Use standard electron beam lithography to write the designed metasurface pattern. After development, use an electron beam evaporator to deposit a 30 nm gold layer on the photoresist to obtain the metasurface structure.

[0008] 3. According to claim 1, a dual-band mode-locked fiber laser based on metasurface polarization control is characterized in that: the golden cross metasurface couples light onto the golden cross metasurface through the fiber collimator 1 and the fiber collimator 2 to ensure that the size of the light spot is smaller than the processed sample area of ​​the golden cross metasurface.

[0009] Preferably, the golden cross metasurface couples light onto the golden cross metasurface through the optical fiber collimator 1 and the optical fiber collimator 2, so as to ensure that the size of the light spot is smaller than the processed sample area of ​​the golden cross metasurface. The transmittance of the golden cross metasurface is related to the polarization state of light, and the output of dual-band mode-locked laser can be achieved by designing the golden cross metasurface structure with different polarization directions and different lengths.

[0010] Preferably, the first loop and the second loop are independent of each other and do not affect each other, and the first loop independently controls the polarization state of light in the corresponding first loop through the polarization controller without affecting the second loop; The second loop independently controls the polarization state of light in the corresponding second loop through the second polarization controller without affecting the first loop.

[0011] Preferably, the pump source 1, the wavelength division multiplexer 1, the erbium-doped optical fiber, the optical fiber isolator 1, the optical fiber coupler 1 and the polarization controller 1 are all coupled via single-mode optical fiber; The second pump source, the second wavelength division multiplexer, the thulium-doped optical fiber, the second optical fiber isolator, the second optical fiber coupler and the second polarization controller are all coupled through single-mode optical fiber.

[0012] Preferably, the first loop is in the 1550 nm band, and the second loop is in the 2000 nm wave.

[0013] Preferably, the transmission directions of the optical signal in the first loop and the second loop are opposite, namely, clockwise in the 1550 nm band and counterclockwise in the 2000 nm band, and the running directions at the common end are the same.

[0014] Preferably, adjusting the tilt angle of the golden cross metasurface is used to achieve different modulation depths of the saturable absorber.

[0015] Therefore, the present invention adopts a dual-band mode-locked fiber laser based on metasurface polarization control with the above structure, which has the following beneficial effects: The dual-band ultrafast laser device of the present invention is based on the metal surface plasmon effect. By preparing a golden cross metasurface mode-locking device, golden cross metasurface structures with inconsistent lengths in the horizontal and vertical directions are designed respectively. By utilizing their polarization and nonlinear saturable absorption characteristics, mode-locked laser output in two bands is achieved simultaneously, and the lasers in the two bands are independent of each other and do not affect each other.

[0016] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic structural diagram of a dual-band mode-locked fiber laser based on metasurface polarization control according to the present invention; Figure 2 This is an electron microscope image of a gold cross metasurface of a dual-band mode-locked fiber laser based on metasurface polarization control according to the present invention; Figure 3 The experimental test results of the transmittance of the golden cross metasurface under different polarizations in a dual-band mode-locked fiber laser based on metasurface polarization control according to the present invention are as follows; Figure 4 This is a spectrum diagram of a dual-band mode-locked fiber laser based on metasurface polarization control according to the present invention; Figure 5 This is a pulse sequence diagram of a dual-band mode-locked fiber laser based on metasurface polarization control according to the present invention; Reference numerals 1. Pump source 1, 2. Wavelength division multiplexer 1, 3. Erbium-doped fiber, 4. Fiber isolator 1, 5. Polarization controller 1, 6. Fiber coupler 1, 7. Wavelength division multiplexer 3, 8. Fiber collimator 1, 9. Golden cross metasurface, 10. Fiber collimator 2, 11. Wavelength division multiplexer 4, 12. Polarization controller 2, 13. Fiber coupler 2, 14. Wavelength division multiplexer 2, 15. Thulium-doped fiber, 16. Fiber isolator 2, 17. Pump source 2. DETAILED DESCRIPTION

[0018] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0019] Unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0020] Example like Figure 1As shown, the present invention provides a dual-band mode-locked fiber laser based on polarization control of a metasurface, comprising: a pump source 1, a wavelength division multiplexer 2, an erbium-doped fiber 3, a fiber isolator 4, a polarization controller 5, a fiber coupler 6, a wavelength division multiplexer 3 7, a fiber collimator 1 8, a golden cross metasurface 9, a fiber collimator 2 10, a wavelength division multiplexer 4 11, a polarization controller 2 12, a fiber coupler 2 13, a wavelength division multiplexer 2 14, a thulium-doped fiber 15, a fiber isolator 2 16, and a pump source 2 17.

[0021] The dual-band mode-locked fiber laser consists of a first loop, a second loop and a common end. The first loop is a loop of the 1550 nm band, and the second loop is a loop of the 2000 nm band. The common end uses a fiber coupler 16 to transfer the 1550 nm band light generated in the first loop and the 2000 nm band light of the second loop through a fiber collimator 210 respectively into the golden cross metasurface 9, which is mainly composed of an upper golden cross structure and a bottom ITO glass.

[0022] The Golden Cross metasurface adopts micro-nano processing technology, which specifically includes the following production steps: S1. Select a glass substrate with an indium tin oxide (ITO) coating as the processing material, and clean the sample in an ultrasonic wave with acetone, isopropyl alcohol solution, and deionized water to remove impurities; S2, drying the cleaned sample substrate with nitrogen, baking at 180° C. for 3 minutes, spin-coating a layer of positron anti-corrosion agent on the substrate, and baking again at 180° C. for 3 minutes; S3. Use standard electron beam lithography to write the designed metasurface pattern. After development, use an electron beam evaporator to deposit a 30 nm gold layer on the photoresist to obtain the metasurface structure.

[0023] The transmission directions of the optical signal in the first loop and the second loop are opposite, that is, clockwise in the first loop and counterclockwise in the second loop, but the transmission directions at the common end portion are the same.

[0024] First, consider the optical path diagram of the first loop. In the present invention, the transmission process of the light of the first loop is as follows: the pump light emitted by the pump source 1 enters the annular optical path through the wavelength division multiplexer 2, and then passes through the erbium-doped fiber 3 for gain amplification, and then passes through the fiber isolator 4, the fiber coupler 6 and the wavelength division multiplexer 4 11 respectively, and then the fiber collimator 2 10 accurately transmits the light to the designed golden cross metasurface 9. After the light and the golden cross metasurface 9 produce saturable absorption, they are coupled to the fiber collimator 8 for circulation to form a fiber loop. The polarization state in the cavity is controlled by the polarization controller 5. When the polarization controller 5 is rotated to the appropriate position, the phase in the loop can be locked and the ultrafast pulse laser can be output. Among them, the fiber isolator 4 can ensure the unidirectional transmission of light in the optical path.

[0025] The optical transmission process of the second loop is similar to that of the first loop. The optical transmission process of the second loop is as follows: the pump light emitted by the pump source 2 17 enters the annular optical path through the wavelength division multiplexer 2 14, and then passes through the thulium-doped optical fiber 15 for gain amplification, and then passes through the optical fiber isolator 2 16, the optical fiber coupler 2 13 and the wavelength division multiplexer 4 11 respectively, and then the optical fiber collimator 2 10 accurately transmits the light to the designed golden cross metasurface 9. After the light and the golden cross metasurface 9 produce saturable absorption, they are coupled to the optical fiber collimator 1 8 for circulation to form a fiber loop. The polarization state in the cavity is controlled by the polarization controller 2 12. When the polarization controller 2 12 is rotated to the appropriate position, the phase in the loop can be locked and the ultrafast pulse laser can be output. Among them, the optical fiber isolator 2 16 can ensure the unidirectional transmission of light in the optical path.

[0026] The first loop and the second loop are independent of each other. The polarization states of the light in the first loop and the second loop can be independently controlled by the polarization controller 1 5 and the polarization controller 2 12 respectively without affecting the other loop.

[0027] Optionally, the fiber coupler 16 and the fiber coupler 2 13 in the embodiment of the present invention are both 10:90 single-mode fiber couplers, the coupling ratio of the fiber coupler 16 and the fiber coupler 13 is 10:90, 10% of the output ports are used for signal monitoring, and the 90% ports are connected to the wavelength division multiplexer to continue circulating in the ring cavity. The 90% output port of the two output ports of the fiber coupler 16 and the fiber coupler 2 13 is connected to the wavelength division multiplexer 4 11 and connected to the entire loop, and the 10% output port is connected to a spectrum analyzer (AQ6370D, Yokogawa) with a resolution of 0.02 nm and an oscilloscope (MSO64, Tektronix) to measure the spectrum and pulse sequence, and the oscilloscope is connected with a photodetector. The wavelength division multiplexer 4 11 is connected to the fiber collimator 2 10, which is used to collimate the light beam and ensure that the laser can effectively hit the golden cross metasurface 9. The polarization state of the light in the first loop and the second loop can be controlled by the polarization controller 1 5 and the polarization controller 2 12 respectively. The polarization controller 1 5 and the polarization controller 2 12 are both three-ring fiber polarization controllers, where the three rings are equivalent to λ / 4, λ / 2 and λ / 4 wave plates respectively, which control the polarization direction, polarization angle and polarization intensity of the light respectively, and the polarization state in the loop can be controlled and the laser energy can be adjusted through fine adjustment.

[0028] Optionally, in this embodiment, the pump source 1 used is a laser with a central wavelength of 980 nm, and the pump source 2 17 is a laser with a central wavelength of 1565 nm; the wavelength division range of the wavelength division multiplexer 2 used is 980 nm / 1550 nm, and the wavelength division ranges of the wavelength division multiplexer 3 7, the wavelength division multiplexer 4 11 and the wavelength division multiplexer 2 14 are all 1550 nm / 2000 nm.

[0029] The present invention realizes the output of mode-locked laser in two bands at the same time. Its working principle is as follows: the present invention is based on the metal surface plasmon effect. In the golden cross metasurface material, the quasi-free electrons in the metal nanostructure resonate with the electromagnetic field and generate two surface plasmon resonance peaks. On this basis, we designed a golden cross metasurface structure with different lengths in the horizontal and vertical polarization directions, so that it can produce a resonance effect on light in different bands.

[0030] See also Figure 2 and Figure 3 ,like Figure 2 This is an electron microscope image of the golden cross metasurface. Figure 3 These are the experimental test results of the transmittance of the golden cross metasurface designed for the present invention under different polarizations. Figure 3It is clearly shown that when different linear polarized lights pass through the golden cross metasurface 9 designed by us, the minimum transmittance is different, reflecting the characteristics of different resonance positions, where x polarization is horizontal polarization direction and y polarization is vertical polarization direction. In order to achieve dual-band (1550 nm and 2000 nm) mode-locked laser output that operates independently at the same time, we focus the clockwise and counterclockwise lasers generated in the first loop and the second loop onto the golden cross metasurface 9 through the fiber collimator 10 respectively. By adjusting the polarization controller 1 5 and polarization controller 2 12 in the first loop and the second loop respectively, when the polarization in the optical path meets the phase locking condition, the golden cross metasurface 9 acts as a saturated absorber, allowing the light with higher energy in the middle to pass through, while the light with lower energy on both sides is absorbed, thereby achieving narrow pulse width and high energy mode-locked pulse laser output.

[0031] It should be noted that by adjusting the polarization controller 212 of the second loop, we observed that there was a certain deviation between the incident y-polarized light and the actual resonance position in the 2000 nm band. The root of this deviation can be traced back to the 2000 nm band of the target design, which was affected by the ITO glass substrate and sample processing errors. Nevertheless, the 2000 nm band is still within the wide range of the resonance peak, ensuring the occurrence of the saturated absorption effect, thus satisfying the basic conditions for mode-locked lasers, and finally achieving mode-locked laser output in the 2000 nm band.

[0032] See also Figure 4 , Figure 4 This is the spectrum of the dual-band mode locking based on polarization control of the metasurface of the present invention. The horizontal axis of the spectrum is wavelength (Wavelength (nm)), and the vertical axis is the light intensity (dBm) on a logarithmic scale. The two center wavelengths are 1560 nm and 1973 nm, respectively, located in the 1550 nm band and the 2000 nm band.

[0033] See also Figure 5 , Figure 5 This is a pulse sequence diagram of dual-band mode locking based on metasurface polarization control in the present invention. The abscissa of the pulse sequence diagram is time (Time (ns)), the ordinate is light pulse intensity (Intensity (au)), and the pulse intervals S1 and S2 between the two bands are both 56.95 ns.

[0034] Different from the existing technology, the present invention uses metasurface technology to achieve multi-band ultrafast laser output with orthogonal polarizations. Compared with traditional two-dimensional materials, its control dimension is wider and the laser output is more stable. The dual-band ultrafast laser based on metasurface polarization control has higher flexibility, providing a new idea for the design of fiber lasers.

[0035] In summary, the dual-band mode-locked fiber laser based on metasurface polarization control of the present invention can realize saturable absorption of light in the two bands of 1550 nm and 2000 nm, and introduces the golden cross metasurface into the common end part of the two-band loop to realize dual-band ultrafast pulse laser output.

[0036] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A dual-band mode-locked fiber laser based on metasurface polarization control, characterized in that: It comprises a first loop, a second loop and a common end, wherein the first loop and the second loop are both connected to the common end; The first loop includes a pump source 1, a wavelength division multiplexer 1, an erbium-doped optical fiber, a fiber isolator 1, a fiber coupler 1 and a polarization controller 1 arranged in a surrounding manner; The second loop includes a second pump source, a second wavelength division multiplexer, a thulium-doped optical fiber, a second optical fiber isolator, a second optical fiber coupler and a second polarization controller arranged in a surrounding manner; The common end includes a wavelength division multiplexer 3, a fiber collimator 1, a fiber collimator 2, a wavelength division multiplexer 4 and a golden cross metasurface which are arranged in sequence.

2. The dual-band mode-locked fiber laser based on metasurface polarization control according to claim 1, characterized in that: The gold cross metasurface adopts micro-nano processing technology, which specifically includes the following production steps: S1. Select a glass substrate with an indium tin oxide (ITO) coating as the processing material, and clean the sample in an ultrasonic wave with acetone, isopropyl alcohol solution, and deionized water to remove impurities; S2, drying the cleaned sample substrate with nitrogen, baking at 180° C. for 3 minutes, spin-coating a layer of positron anti-corrosion agent on the substrate, and baking again at 180° C. for 3 minutes; S3. Use standard electron beam lithography to write the designed metasurface pattern. After development, use an electron beam evaporator to deposit a 30 nm gold layer on the photoresist to obtain the metasurface structure.

3. The dual-band mode-locked fiber laser based on metasurface polarization control according to claim 1, characterized in that: The golden cross metasurface couples light onto the golden cross metasurface through the optical fiber collimator 1 and the optical fiber collimator 2, so as to ensure that the size of the light spot is smaller than the processed sample area of ​​the golden cross metasurface.

4. The dual-band mode-locked fiber laser based on metasurface polarization control according to claim 1, characterized in that: The first loop and the second loop are independent of each other and do not affect each other, and the first loop independently controls the polarization state of light in the first loop through the polarization controller without affecting the second loop; The second loop independently controls the polarization state of light in the corresponding second loop through the second polarization controller without affecting the first loop.

5. The dual-band mode-locked fiber laser based on metasurface polarization control according to claim 1, characterized in that: The pump source 1, the wavelength division multiplexer 1, the erbium-doped optical fiber, the optical fiber isolator 1, the optical fiber coupler 1 and the polarization controller 1 are all coupled through single-mode optical fiber; The second pump source, the second wavelength division multiplexer, the thulium-doped optical fiber, the second optical fiber isolator, the second optical fiber coupler and the second polarization controller are all coupled through single-mode optical fiber.

6. The dual-band mode-locked fiber laser based on metasurface polarization control according to claim 1, characterized in that: The first loop is in the 1550 nm band, and the second loop is in the 2000 nm band.

7. The dual-band mode-locked fiber laser based on metasurface polarization control according to claim 1, characterized in that: The transmission directions of the optical signal in the first loop and the second loop are opposite, that is, clockwise in the first loop and counterclockwise in the second loop, and the running directions at the common end are the same.

8. The dual-band mode-locked fiber laser based on metasurface polarization control according to claim 1, characterized in that: The tilt angle of the golden cross metasurface is adjusted to achieve different modulation depths of the saturable absorber.

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