CN117080850A - Wavelength-tunable vortex fiber laser - Google Patents
Wavelength-tunable vortex fiber laser Download PDFInfo
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/10061—Polarization control
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
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- H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
- H01S3/067—Fibre lasers
- H01S3/06791—Fibre ring lasers
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/08—Construction or shape of optical resonators or components thereof
- H01S3/08013—Resonator comprising a fibre, e.g. for modifying dispersion or repetition rate
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Abstract
The application provides a wavelength-tunable vortex fiber laser, which emits Gaussian beams from a pumping source, the Gaussian beams are coupled into a laser cavity by a wavelength division multiplexer, the beams sequentially pass through an isolator, a tunable filter, a polarization controller, a circulator and a collimator after gain is provided by a non-polarization-maintaining single-mode doped fiber, the beams are focused on an ultra-structured surface by a lens, the ultra-structured surface converts incident beam parts into vortex beams which are mutually orthogonal with the polarization state of the Gaussian beams, the ultra-structured surface reflects the beams to a polarization beam splitter, the polarization beam splitter outputs the orthogonal vortex beams to the outside of the cavity, the Gaussian beams with the same polarization are coupled back into the fiber by the collimator, and the Gaussian beams continue to circulate in a loop in the laser cavity. The application adopts a 9-shaped laser cavity structure, and can generate vortex beams with wider tunable wavelength range by combining with the super-structured surface in the cavity, and the generated vortex beams have controllable topological charge number and higher purity.
Description
Technical Field
The application relates to the technical field of lasers, in particular to a wavelength-tunable vortex fiber laser.
Background
The vortex beam (OAM beam) has the characteristics of unique spiral wave front, annular intensity distribution, carrying orbital angular momentum and the like, and has wide application prospect in the fields of high-capacity optical communication, super-resolution microscopic imaging, particle manipulation and the like. At present, the generation of OAM light beams has become a hot spot for light field regulation research. The conventional method generates an OAM beam outside a laser cavity through devices such as a spiral phase plate and the like, and is often limited by unstable transmission and lower mode purity.
The optical super-structured surface is an emerging artificial material with sub-wavelength size, and has the advantages of small volume, large control freedom, low loss, easy integration and the like. The optical fiber can effectively adjust the degrees of freedom such as amplitude, phase and polarization state of an optical field in a wider wavelength range, and is widely applied to the fields of sensing, imaging, optical communication, holography and the like. Recent studies have shown that vortex fiber lasers based on intra-cavity super-structured surfaces can achieve OAM mode purity of over 90% higher than that of OAM beams generated extracavity using the same super-structured surface. However, previous studies have been limited to polarization maintaining optical fibers and linear cavity types, which have high cost, and the linear cavity structure is not flexible enough, which limits practical applications to some extent.
In addition, wavelength tuning capability is an important measure of vortex-induced optical lasers. The tuning range of the existing vortex fiber laser based on the traditional mode conversion device is limited.
Disclosure of Invention
In view of this, the embodiment of the application provides a wavelength tunable vortex fiber laser, so as to eliminate or improve the bottleneck of limited application scenarios caused by narrow wavelength tuning range, single laser cavity structure and high cost of polarization maintaining fiber lasers in the prior art. The laser is based on a 9-shaped laser cavity structure, can be suitable for different application scenes, and has the advantages of high efficiency, high gain and low cost.
The application relates to a wavelength tunable vortex fiber laser, which is characterized in that the laser comprises:
a pumping source for emitting a light beam of a specific wavelength and supplying energy;
the wavelength division multiplexer is connected with the pump source through a non-polarization-maintaining single-mode fiber and is used for coupling a light beam emitted by the pump source into a laser cavity;
the non-polarization-maintaining single-mode doped optical fiber is connected with the wavelength division multiplexer through a non-polarization-maintaining single-mode optical fiber and is used as a gain medium of the laser;
the isolator is connected with the non-polarization-maintaining single-mode doped optical fiber through a non-polarization-maintaining single-mode optical fiber and is used for controlling unidirectional transmission of light beams;
the tunable filter is connected with the isolator through a non-polarization-maintaining single-mode fiber and is used for adjusting the wavelength of the light beam;
the polarization controller is connected with the tunable filter through a non-polarization-maintaining single-mode fiber and is used for controlling the polarization direction of the light beam;
the circulator comprises three ports, wherein the first port is connected with the polarization controller through a non-polarization-maintaining single-mode fiber, the second port is connected with the collimator through the non-polarization-maintaining single-mode fiber, the third port is connected with the wavelength division multiplexer through the non-polarization-maintaining single-mode fiber, and the circulator is used for controlling the transmission direction of light beams;
a collimator connected to the circulator at the second port through a non-polarization maintaining single mode fiber, the collimator being configured to couple a light beam in an optical fiber with a light beam in free space within the laser cavity;
a polarization beam splitter disposed in a free space within the laser cavity for splitting two orthogonally polarized light beams passing through the polarization beam splitter into two different propagation directions;
a lens disposed in a free space within the laser cavity, the lens being configured to focus the light beam emitted through the collimator onto the super-structured surface, and then to change the light beam reflected from the super-structured surface into a parallel light beam;
the super-structure surface is arranged in the free space in the laser cavity, and the collimator is coupled to the light beam in the free space in the laser cavity, sequentially passes through the polarization beam splitter and the lens and reaches the super-structure surface; the super-structured surface receives an incident light beam and reflects a Gaussian light beam with a first polarization direction and a vortex light beam with a second polarization direction; the first polarization direction and the second polarization direction are mutually orthogonal; the structure of the super-structured surface sequentially comprises from top to bottom: the device comprises a nano metal rod array, an insulator spacer layer and a bottom metal substrate; the vortex light beam reflected by the super-structure surface is a light beam with orbital angular momentum and topological charge number being an integer;
the polarization beam splitter is arranged between the collimator and the lens, the lens is arranged between the polarization beam splitter and the super-structure surface, the polarization beam splitter completely transmits Gaussian beams in a first polarization direction emitted by the collimator, the polarization beam splitter receives the beams reflected by the super-structure surface, couples out vortex beams in the beams to a laser cavity, transmits the Gaussian beams back to the collimator, transmits the Gaussian beams to the circulator through the collimator, and transmits the Gaussian beams to the wavelength division multiplexer through the third port.
In some embodiments of the application, the non-polarization maintaining single mode doped fiber is an ytterbium doped fiber.
In some embodiments of the application, the non-polarization maintaining single mode doped fiber is an erbium doped fiber.
In some embodiments of the application, the nano-metal rod material is gold, the insulator spacer layer material is silicon dioxide, and the bottom metal substrate material is gold.
In some embodiments of the application, the nano-metal rod material is silver or aluminum, the insulator spacer layer material is silicon dioxide, and the bottom metal substrate material is silver or aluminum.
In some embodiments of the present application, in the super-structured surface structure, the nano-metal rod array has a thickness of 20-100 nm, the insulator spacer layer has a thickness of 20-150 nm, and the bottom metal substrate has a thickness of more than 100nm.
In some embodiments of the application, the period of the super-structured surface is less than the quotient of the vacuum wavelength of the normal incident beam and the refractive index of the insulating layer.
In some embodiments of the application, the long or short axes of the nano-metal rods in the super-structured surface are at 45 ° to the azimuth angle of the first polarization direction.
In some embodiments of the application, the arrays of nano-metal rods in the super-structured surface are arranged in 8 groups, the 8 groups of nano-metal rod arrays having four long and wide dimensional arrangements, wherein the 4 groups of nano-metal rod arrays differ from the other 4 groups of nano-metal rod arrays by 90 ° in azimuth, and the increment of cross-polarization phase shift between adjacent groups of nano-metal rod array elements is pi/4.
In some embodiments of the application, the pump source is 976nm continuous light, and the wavelength division multiplexer operates at 976/1030nm or 976/1550nm.
The application has the advantages that:
the application provides a wavelength tunable vortex fiber laser, which emits Gaussian beams from a pumping source, the Gaussian beams are coupled into a laser cavity through a wavelength division multiplexer, and the Gaussian beams are emitted to a lens in a free space after passing through elements such as a non-polarization-maintaining single-mode doped fiber, an isolator, a tunable filter, a polarization controller, a circulator, a collimator and the like, and then focused on an ultra-structured surface. The super-structured surface converts part of the incident light beam into vortex light beams which are mutually orthogonal with the polarization state of the Gaussian light beam, the vortex light beams are output to the outside of the cavity through the polarization beam splitter, meanwhile, the Gaussian light beam is transmitted back to the collimator, and the transmission direction of the returned Gaussian light beam is controlled through the circulator, so that the Gaussian light beam circulates in the laser cavity. The application relates to a laser cavity structure which is applicable to more application scenes, and vortex light beams with wider tunable wavelength range and higher purity are output through a laser cavity structure and an optical super-structure surface which are skillfully designed.
Additional advantages, objects, and features of the application will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the application. The objectives and other advantages of the application may be realized and attained by the structure particularly pointed out in the written description and drawings.
It will be appreciated by those skilled in the art that the objects and advantages that can be achieved with the present application are not limited to the above-described specific ones, and that the above and other objects that can be achieved with the present application will be more clearly understood from the following detailed description.
Drawings
The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate and together with the description serve to explain the application. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the application. Corresponding parts in the drawings may be exaggerated, i.e. made larger relative to other parts in an exemplary device actually manufactured according to the present application, for convenience in showing and describing some parts of the present application. In the drawings:
FIG. 1 is a schematic diagram of a wavelength tunable vortex fiber laser according to an embodiment of the present application.
Fig. 2 is a schematic diagram of the operation of GSP super-structure surface of MIM structure with orthogonal linear polarization conversion and its unit structure according to an embodiment of the present application.
FIG. 3 shows the reflectivity R of cross-polarized reflected light of an ultra-structured surface element in an embodiment of the application cr And phase offsetA simulated graph; topological charge number l=1 and topological charge number l= -1.
Fig. 4 shows tunable spectra, threshold pump power, and OAM mode purity for a laser in the 1020nm to 1060nm wavelength range, in accordance with an embodiment of the present application; the intensity profile of the OAM beam is output within the cavity of the laser in the wavelength range 1020nm to 1060 nm.
Detailed Description
The present application will be described in further detail with reference to the following embodiments and the accompanying drawings, in order to make the objects, technical solutions and advantages of the present application more apparent. The exemplary embodiments of the present application and the descriptions thereof are used herein to explain the present application, but are not intended to limit the application.
It should be noted here that, in order to avoid obscuring the present application due to unnecessary details, only structures and/or processing steps closely related to the solution according to the present application are shown in the drawings, while other details not greatly related to the present application are omitted.
It should be emphasized that the term "comprises/comprising" when used herein is taken to specify the presence of stated features, elements, steps or components, but does not preclude the presence or addition of one or more other features, elements, steps or components.
It is also noted herein that the term "coupled" may refer to not only a direct connection, but also an indirect connection in which an intermediate is present, unless otherwise specified.
Hereinafter, embodiments of the present application will be described with reference to the accompanying drawings. In the drawings, the same reference numerals represent the same or similar components, or the same or similar steps.
A fiber laser is a laser that uses an optical fiber as a laser medium and an optical transmission channel, and amplifies and transmits laser energy through the optical fiber. Fiber lasers are typically composed of three main components: pump source, laser medium, optical resonant cavity. The pump source in the fiber laser is used to inject energy into the fiber, exciting atomic or molecular transitions in the lasing medium, thereby producing laser amplification. The laser medium is typically an elongated optical fiber made of a material having a high refractive index. The laser medium is internally subjected to special doping or optical structural design, so that the light energy is stimulated to radiate and amplify in the fiber core, thereby generating laser and determining the wavelength of output laser. The optical resonant cavity in the fiber laser consists of a wavelength division multiplexer, an isolator and other devices and forms a 9-shaped loop to realize laser amplification and feedback.
An embodiment of the present application provides a wavelength tunable vortex fiber laser, as shown in fig. 1, comprising: the device comprises a pumping source 1 for emitting a light beam with a specific wavelength, a wavelength division multiplexer 2, a non-polarization-maintaining single-mode doped optical fiber 3, an isolator 4, a tunable filter 5, a polarization controller 6, a circulator 7 and a collimator 8, wherein the devices are connected through the non-polarization-maintaining single-mode optical fiber; the laser comprises a laser cavity, a collimator 8, a polarization beam splitter 9, a lens 10 and an ultra-structure surface 11, wherein the polarization beam splitter 9, the lens 10 and the ultra-structure surface 11 are positioned in the free space of the laser cavity, and the collimator 8 is a medium for coupling an optical fiber passage and the free space, and all the devices form a 9-shaped annular cavity structure of the laser.
The wavelength division multiplexer 2 couples a Gaussian beam emitted by the pump source 1 into a laser cavity, the Gaussian beam sequentially passes through the non-polarization-maintaining single-mode doped optical fiber 3, the isolator 4, the tunable filter 5, the polarization controller 6, the circulator 7 and the collimator 8, the collimator 8 couples the Gaussian beam into a free space of non-optical fiber transmission, the polarization state of the Gaussian beam is x polarization, the Gaussian beam sequentially passes through the polarization beam splitter 9 and the lens 10 in the free space to reach the super-structured surface 11, the super-structured surface 11 reflects a modulated beam, the modulated beam is split into two beams at the polarization beam splitter 9 through the lens 10, one beam is a y polarized vortex beam, and the vortex beam is output from the output end of the laser cavity. The other beam is an x-polarized gaussian beam which is transmitted by the polarizing beam splitter 9 to the collimator 8, from the collimator 8 back into the fibre path to the circulator 7.
A gaussian beam is a special type of laser beam whose intensity distribution exhibits a gaussian distribution in the lateral direction. The Gaussian beam involved in this embodiment is a single mode fiber supported LP 01 A mode.
The vortex beam is a beam with orbital angular momentum (orbital angular momentum, OAM). Unlike conventional plane waves or gaussian beams, a vortex beam has a helical phase structure, similar to a vortex or helix. The orbital angular momentum describes the rotational nature of the beam envelope, and the phase structure of the swirling beam is achieved by introducing a helical phase in the lateral direction.
The circulator 7 includes three ports, namely, a port 71 connected to the polarization controller 6, a port 72 connected to the collimator 8, and a port 73 connected to the wavelength division multiplexer 2. The circulator 7 has two transport directions, a first transport direction: from port 71 to port 72, second transmission direction: from port 72 to port 73. The gaussian beam from free space back to the intra-cavity optical fibre path reaches the port 72 of the circulator 7 and will be transmitted to the wavelength division multiplexer 2 by the second transmission direction, enabling self-consistent transmission of the laser. Self-consistent delivery means that the state of the beam at each location within the laser cavity is the same as that at that location in the previous intra-cavity cycle.
Wherein the non-polarization maintaining single-mode doped fiber 3 is capable of providing a desired gain for laser oscillation.
Wherein the isolator 4 is used to control the unidirectional transmission of the light beam.
The tunable filter 5 is used for adjusting the wavelength of the light beam, so as to continuously adjust the working wavelength of the laser in a broadband range.
Wherein the polarization controller 6 is used for controlling the polarization direction of the light beam, so as to ensure that the light beam incident on the super-structure surface has x polarization.
The polarizing beam splitter 9 is used for splitting two beams which are subjected to the polarizing beam splitter and are subjected to orthogonal linear polarization into different propagation directions, and the polarizing beam splitter 9 can completely transmit the beams in the x polarization direction and simultaneously reflect and couple the vortex beams in the y polarization out of the laser cavity.
Wherein a lens 10 is arranged in free space within the laser cavity between the polarizing beam splitter 9 and the super-structured surface 11, the lens 10 focusing the beam of light coupled by the collimator 8 into the x-polarization direction in free space onto the super-structured surface 11.
The super-structured surface 11 is disposed in a free space in the laser cavity, the light beam in the x polarization direction is focused on the super-structured surface 11, the gaussian light beam in the x polarization direction and the vortex light beam in the y polarization direction are reflected, the gaussian light beam in the x polarization direction and the vortex light beam in the y polarization direction are mutually orthogonal, and the vortex light beam is a light beam with orbital angular momentum and an integer of topological charge. The super-structured surface 11 is a plasmon super-structured surface, and adopts a metal-insulator-metal structure, and comprises, from top to bottom: the device comprises a nano metal rod array, an insulator spacer layer and a bottom metal substrate.
In some embodiments of the application, the non-polarization maintaining single mode doped fiber is an ytterbium doped fiber.
In some embodiments of the application, the non-polarization maintaining single mode doped fiber is an erbium doped fiber.
In some embodiments of the present application, the nano-metal rod material is gold, the insulator spacer layer material is silicon dioxide, and the bottom metal substrate material is gold.
In some embodiments of the application, the nano-metal rod material is silver or aluminum, the insulator spacer layer material is silicon dioxide, and the bottom metal substrate material is silver or aluminum.
In some embodiments of the application, the nanostructure surface structure has an array of nano-metal rods 80nm thick, an insulator spacer layer 110nm thick, and a bottom metal substrate 130nm thick.
In some embodiments of the application, the period of the super-structured surface is 550nm. The period of the super-structured surface refers to the pitch or periodicity of the recurring structural elements on the super-structured surface. A super-structured surface is a surface with micro-nano scale structures in which the structural units are arranged in a specific manner to form periodic features. The pitch of these building blocks is the period of the super-structured surface. In the super-structured surface design, the choice of period affects the specific optical effect of the super-structured surface. The phase and amplitude response of the super-structured surface can be adjusted by using the period, so that the precise control of the characteristics of scattering, absorption and the like is realized.
In some embodiments of the application, the long or short axis direction of the nano-metal rods in the super-structured surface is at 45 ° to the azimuth angle of the x-polarization direction.
In some embodiments of the application, the arrays of nano-metal rods in the super-structured surface are arranged in 8 groups, the 8 groups of nano-metal rod arrays having four long and wide dimensional arrangements, wherein the 4 groups of nano-metal rod arrays are 90 ° out of azimuth from the other 4 groups of nano-metal rod arrays.
In some embodiments of the application, the cross-polarization phase shift between adjacent nanorod array elements increases by pi/4 in 8 groups of nanorod arrays in the super-structured surface.
In some embodiments of the application, the pump source is 976nm continuous light and the wavelength division multiplexer operates at 976/1030nm or 976/1550nm.
Alternatively, another embodiment of the present application provides a wavelength tunable vortex fiber laser, as shown in fig. 1, comprising: pump source 1, wavelength division multiplexer 2, non-polarization preserving single mode doped fiber 3, isolator 4, tunable filter 5, polarization controller 6, circulator 7, collimator 8, polarization beam splitter 9, lens 10, and super-structured surface 11. The specific technical scheme of the embodiment is as follows:
in recent years, vortex beams (also called OAM beams) have great application prospects in the fields of high-capacity optical communication, super-resolution microscopic imaging, particle manipulation and the like due to the characteristics of unique spiral wave fronts, annular intensity distribution, carrying topological charges and the like, and the generation of OAM structure beams has become a research hot spot in the current optical field regulation direction. Outside the cavity, the incident beam is typically converted from gaussian mode to OAM mode using a mode conversion element such as a spiral phase plate, fork grating, or the like. Compared with laser extra-cavity optical field modulation, the strategy of placing the mode conversion element in the cavity to form the vortex laser source has the advantages of high power, high purity, usability and the like, and can construct an OAM structure light source which is more efficient, compact and user-friendly. Compared with the traditional solid-state laser, the fiber laser has the advantages of easy alignment, high gain, low cost, no water cooling (or simple water cooling) and the like. However, conventional vortex fiber lasers have limited wavelength tuning ranges (typically no more than 35 nm) due to the narrow operating bandwidth and/or high insertion loss of the mode-converting element.
The optical super-structured surface is used as an emerging artificial material with sub-wavelength size, has the unique advantages of small volume, large regulation and control degree of freedom, low loss and easy integration, can effectively regulate the degree of freedom of the amplitude, phase, polarization state and the like of an optical field in a wider wavelength range, and is widely applied to the research of the leading edge fields of sensing, imaging, optical communication, holography and the like. In particular, a reflective Gap Surface Plasmon (GSP) super-structured surface composed of a metal-insulator-metal (MIM) structure can form Fabry-Perot cavity resonance between a metal nano rod and an underlying metal substrate, greatly enhances the interaction of light and substances and generates local strong field enhancement, can theoretically realize 100% of light beam conversion efficiency without damage, and is likely to be an ideal choice for mode adjustment in a cavity of a fiber laser.
Although the method of generating an OAM beam as a mode converter by means of a super-structured surface or the like outside the laser cavity is straightforward, this approach is often limited by factors such as unstable transmission, low mode purity, and cannot be used directly as a light source for the user. Due to the mode filtering mechanism of the laser resonant cavity, the mode purity of the OAM beam can be effectively improved. In addition, the efficiency loss caused by the super-structured surface can also be compensated by the laser gain due to the continuous gain amplification within the cavity.
Previously, researchers have demonstrated the diversity regulatory capability of the intracavity super-structured surface to the optical field by inserting dielectric/plasmon super-structured surfaces in solid state lasers, enabling high purity and customizable OAM light sources. Fiber lasers have the advantages of no need for water cooling (or ease of cooling), high efficiency, high gain, and low cost compared to solid state lasers. Furthermore, LP 01 The modes are fundamental modes of light propagation in a single mode fiber (approximately equivalent to a gaussian beam in free space), pure LP in a single mode fiber 01 The mode ensures the ideal gaussian mode morphology incident on the super-structured surface.
The laser resonant cavity in the prior art is limited to a linear cavity, and polarization maintaining optical fibers are adopted for connection among optical fiber devices, so that the practical application is limited to a certain extent. The embodiment of the application adopts the vortex laser with the annular cavity or other structures, and uses the non-polarization maintaining fiber for connection, thereby not only reducing the cost of the laser, but also being more suitable for different application scenes.
Meanwhile, the wavelength tuning capability is also an important index for measuring the structural light laser. The broadband tunable structured light source is expected to find wide application in the fields of optical communication and laser processing. Vortex fiber lasers based on conventional mode-converting devices (e.g., spatial light modulators, long period fiber gratings, mode-selective couplers) have tuning ranges of about 3nm to 35nm. Whereas GSP ultra-structured surfaces have proven to be capable of generating OAM beams over a wavelength range exceeding 100nm, as a modulating component within the laser cavity, with the potential to realize broadband tunable light sources.
The embodiment of the application provides a 9-shaped cavity wavelength tunable vortex fiber laser based on a gap surface plasmon super-structured surface and a non-polarization-maintaining single-mode fiber, which can directly generate high-purity + -1-order OAM light beams in a wide wavelength range from 1020nm to 1060 nm. The super-structured surface is used as an intra-cavity mode conversion device, and a wavelength tunable filter is utilized to continuously adjust the working wavelength of the laser in a broadband range, so that the system scheme for controlling the structural beam at the source of the laser is enriched.
The GSP super structure surface adopted by the embodiment of the application operates in a reflection mode, so the embodiment provides the 9-shaped cavity fiber laser cavity configuration shown in figure 1. The optical fiber devices in the laser cavity are connected by standard non-polarization-maintaining single-mode optical fibers. 976nm continuous light is used as a pump source and coupled into the laser cavity through a 976/1030nm wavelength division multiplexer. Ytterbium-doped fibers are used to provide the gain required for laser oscillation. The fiber optic isolator ensures unidirectional transmission (arrow direction) of the light beam. The three port fiber optic ring has a specific direction of transmission: a transmission direction from port 71 to port 72, and a transmission direction from port 72 to port 73. The collimator acts as a medium for the coupling of the optical fibers to free space, and the beam coupled from the collimator to free space is a parallel gaussian beam. By rotating the polarization controller, the polarization state of the laser cavity can be adjusted so that the gaussian beam is transmitted completely directly through the polarizing beam splitter with minimal loss (e.g., x-polarization) and the parallel beam is focused by the acromatic lens onto the super-structured surface. The focusing light spot size is smaller than the super-surface size, so that the light beam is fully regulated and controlled by the super-structured surface.
The beam reflected from the super-structured surface comprises two parts: OAM beams on cross-polarized (e.g., y-polarized) channels and residual beams on co-polarized (e.g., x-polarized) channels that are not converted. The polarization states of the two beams are orthogonal to each other, so that the y-polarized OAM beam can be reflected by the polarizing beam splitter and coupled out of the cavity, while the residual beam is transmitted by the polarizing beam splitter and re-coupled back into the fiber optic path. The loop returns to the main loop from port 72 to port 73, and self-consistent transmission of the 9-cavity laser is achieved. Self-consistent delivery means that the state of the beam at each location within the laser cavity is the same as that at that location in the previous intra-cavity cycle. Because the designed super-structured surface can generate OAM light beams in a wider wavelength range (more than 100 nm), the wavelength-tunable laser source is hopeful to be realized, and the working wavelength of the laser can be dynamically adjusted through a wavelength-tunable filter in the cavity.
The working principle of a GSP super-structured surface with orthogonal linear polarization converted MIM (Metal-insulator-Metal) structure is shown in fig. 2 a. On the cross polarization channel, the GSP (Gap-surface plasmon) super-structured surface can effectively convert Gaussian beams into OAM (Orbital angular momentum orbital angular momentum) beams, and the conversion efficiency is controllable.
As shown in FIG. 2b, the GSP super structure surface is composed of an Au nano rod array and SiO 2 The spacer layer and the bottom Au substrate are formed, and the thickness of three layers from top to bottom is t respectively 1 ＝80nm、t 2 =110 nm and t 3 130nm, wherein the Au nanorods have an azimuthal angle of 45 ° to the x-axis. The period of the super-structured surface is p=550 nm. Besides the plasmon resonance generated by the Au nanorod array, the underlying Au substrate also introduces Fabry-Perot-like cavity resonance. Compared with the single-layer plasma super-structured surface, the polarization conversion efficiency is greatly improved. Since the cross polarized light generated by the super-structured surface is coupled out of the cavity, the cross polarized light can be regarded as a loss channel in the laser cavity, and the excessively high polarization conversion efficiency is equivalent to the increase of loss in the cavity, so that the threshold pumping power required by the laser to start oscillation is increased. In order to ensure that there is enough co-polarized residual beam components in the cavity to perform feedback oscillation while outputting an efficient OAM beam on the cross-polarized channels, proper polarization conversion efficiency needs to be selected to achieve balance.
With limited use ofMeta analysis method for calculating cross polarization reflectivity R of super-structured surface unit under combination (m, n) of length and width of different nano rods cr And phase offsetAnd four different nanorod sizes were selected from them. As shown in fig. 3a, the cross-polarization reflectivity of four groups of super-structured surface elements is around 40% over a broad band range of 1000-1100 nm, and the cross-polarization phase shift delta between adjacent elements is pi/4 as shown in fig. 3 b. Because the nano rods with 90 degrees of azimuth angle difference have pi phase difference, the four nano rods are simultaneously rotated by 90 degrees around the z axis, and full-phase regulation and control within the range of 2 pi can be realized. By arranging these 8 sets of super-structured surface elements clockwise or counterclockwise with pi/4 phase shift increments, two super-structured surfaces are designed, which can generate OAM beams with topology charges of l=1 and-1, respectively. The designed GSP super structure surface can operate the light field in a broadband range, thereby realizing the wavelength adjustable output of the OAM light beam in the fiber laser. Fig. 3c and 3d are scanning electron microscope images of a super-structured surface processed by electron beam lithography.
Therefore, the embodiment of the application takes the super-structured surface as an intracavity optical field regulating device and continuously regulates the working wavelength of the fiber laser through the tunable filter. As shown in fig. 4a, a continuously tunable OAM beam can be obtained from within the cavity at wavelengths of 1020nm to 1060 nm. The optical fiber laser based on the super-structured surface assist proposed in the present embodiment exhibits a larger tuning range (40 nm) than the tuning range (typically 3nm to 35 nm) of the vortex optical fiber laser based on the conventional mode conversion element. The relatively narrow tunable range of lasers compared to GSP super-structured surfaces is limited primarily by the limited gain spectrum of single-mode ytterbium-doped fibers, the limited operating bandwidth of intracavity elements (e.g., isolators, circulators, etc.).
Furthermore, the variation in the operating efficiency of the devices in the cavity at different wavelengths results in a variation in the total loss in the cavity, and therefore the pump power required to start oscillation of the laser at each wavelength fluctuates. Fig. 4b shows that the laser has the greatest net gain in the cavity near 1040nm, while the highest pump injection power is required to reach the oscillation threshold at 1060 nm. The mode purity of the OAM beam can be obtained by demodulating the emitted OAM beam by a Spatial Light Modulator (SLM) loaded with helical phases of different orders, and experiments show that the change of the operating wavelength hardly affects the mode purity (about 90%) of the output OAM beam. Fig. 4c and fig. 4c show the intensity distribution (first row) and fork interference fringes (second row) of the OAM beam obtained at 1020nm to 1060nm, the central dark spot of annular intensity originating from the singular point formed by the spiral phase envelope of the OAM beam. The OAM beam and the Gaussian beam are interfered to form fork-shaped interference fringes, and for topological charges with opposite signs, the opening directions of the fork-shaped patterns are opposite, and the topological charge number of the OAM beam can be obtained by observing the fringe number at the fork-shaped openings.
In summary, the present application provides a wavelength tunable vortex fiber laser, the laser emits a gaussian beam from a pump source, the gaussian beam is coupled into a laser cavity by a wavelength division multiplexer, the beam is provided with gain by a non-polarization-maintaining single-mode doped fiber, and then sequentially passes through an isolator, a tunable filter, a polarization controller, a circulator and a collimator, the beam is focused on an ultra-structured surface by a lens, the ultra-structured surface converts an incident beam part into a vortex beam orthogonal to the polarization state of the gaussian beam, the ultra-structured surface reflects the beam to a polarization beam splitter, the polarization beam splitter outputs the orthogonal polarized vortex beam to the outside of the cavity, the gaussian beam with the same polarization is coupled back into the fiber by the collimator, and the gaussian beam continues to circulate in a loop in the laser cavity. The application designs a 9-shaped laser cavity structure, and can generate vortex beams with wider tunable wavelength range by combining with the ultra-structured surface in the cavity, and the generated vortex beams have controllable topological charge number and higher purity.
Those of ordinary skill in the art will appreciate that the various illustrative components, systems, and methods described in connection with the embodiments disclosed herein can be implemented as hardware, software, or a combination of both. The particular implementation is hardware or software dependent on the specific application of the solution and the design constraints. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present application. When implemented in hardware, it may be, for example, an electronic circuit, an Application Specific Integrated Circuit (ASIC), suitable firmware, a plug-in, a function card, or the like. When implemented in software, the elements of the application are the programs or code segments used to perform the required tasks. The program or code segments may be stored in a machine readable medium or transmitted over transmission media or communication links by a data signal carried in a carrier wave.
It should be understood that the application is not limited to the particular arrangements and instrumentality described above and shown in the drawings. For the sake of brevity, a detailed description of known methods is omitted here. In the above embodiments, several specific steps are described and shown as examples. However, the method processes of the present application are not limited to the specific steps described and shown, and those skilled in the art can make various changes, modifications and additions, or change the order between steps, after appreciating the spirit of the present application.
In this disclosure, features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments.
The above description is only of the preferred embodiments of the present application and is not intended to limit the present application, and various modifications and variations can be made to the embodiments of the present application by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.
Claims (10)
1. A wavelength tunable vortex fiber laser, the laser comprising:
a pumping source for emitting a light beam of a specific wavelength and supplying energy;
the wavelength division multiplexer is connected with the pump source through a non-polarization-maintaining single-mode fiber and is used for coupling a light beam emitted by the pump source into a laser cavity;
the non-polarization-maintaining single-mode doped optical fiber is connected with the wavelength division multiplexer through a non-polarization-maintaining single-mode optical fiber and is used as a gain medium of the laser;
the isolator is connected with the non-polarization-maintaining single-mode doped optical fiber through a non-polarization-maintaining single-mode optical fiber and is used for controlling unidirectional transmission of light beams;
the tunable filter is connected with the isolator through a non-polarization-maintaining single-mode fiber and is used for adjusting the wavelength of the light beam;
the polarization controller is connected with the tunable filter through a non-polarization-maintaining single-mode fiber and is used for controlling the polarization direction of the light beam;
the circulator comprises three ports, wherein the first port is connected with the polarization controller through a non-polarization-maintaining single-mode fiber, the second port is connected with the collimator through the non-polarization-maintaining single-mode fiber, the third port is connected with the wavelength division multiplexer through the non-polarization-maintaining single-mode fiber, and the circulator is used for controlling the transmission direction of light beams;
a collimator connected to the circulator at the second port through a non-polarization maintaining single mode fiber, the collimator being configured to couple a light beam in an optical fiber with a light beam in free space within the laser cavity;
a polarization beam splitter disposed in a free space within the laser cavity for splitting two orthogonally polarized light beams passing through the polarization beam splitter into two different propagation directions;
a lens disposed in a free space within the laser cavity, the lens being configured to focus the light beam emitted through the collimator onto the super-structured surface, and then to change the light beam reflected from the super-structured surface into a parallel light beam;
the super-structured surface is arranged in a free space in the laser cavity, the collimator is coupled to the light beam in the laser cavity, and the light beam sequentially passes through the polarization beam splitter and the lens to reach the super-structured surface; the super-structured surface receives an incident light beam and reflects a Gaussian light beam with a first polarization direction and a vortex light beam with a second polarization direction; the first polarization direction and the second polarization direction are mutually orthogonal; the structure of the super-structured surface sequentially comprises from top to bottom: the device comprises a nano metal rod array, an insulator spacer layer and a bottom metal substrate; the vortex light beam reflected by the super-structure surface is a light beam with orbital angular momentum and topological charge number being an integer;
the polarization beam splitter is arranged between the collimator and the lens, the lens is arranged between the polarization beam splitter and the super-structure surface, the polarization beam splitter completely transmits Gaussian beams in a first polarization direction emitted by the collimator, the polarization beam splitter receives the beams reflected by the super-structure surface, couples out vortex beams in the beams to a laser cavity, transmits the Gaussian beams back to the collimator, transmits the Gaussian beams to the circulator through the collimator, and transmits the Gaussian beams to the wavelength division multiplexer through the third port.
2. The wavelength tunable vortex fiber laser of claim 1 wherein the non-polarization maintaining single mode doped fiber is an ytterbium doped fiber.
3. The wavelength tunable vortex fiber laser of claim 1 wherein the non-polarization maintaining single mode doped fiber is an erbium doped fiber.
4. The wavelength tunable vortex fiber laser of claim 1 wherein the nano metal rod material is gold, the insulator spacer layer material is silicon dioxide, and the bottom metal substrate material is gold.
5. The wavelength tunable vortex fiber laser of claim 1 wherein the nano metal rod material is silver or aluminum, the insulator spacer layer material is silicon dioxide, and the bottom metal substrate material is silver or aluminum.
6. The wavelength tunable vortex fiber laser of claim 1 wherein in the super-structured surface structure, the nano-metal rod array thickness is 20-100 nm, the insulator spacer layer thickness is 20-150 nm, and the bottom metal substrate thickness is greater than 100nm.
7. The wavelength tunable vortex fiber laser of claim 1 wherein the period of the super-structured surface is less than the quotient of the normal incidence beam vacuum wavelength and the refractive index of the insulating layer.
8. The wavelength tunable vortex fiber laser of claim 1 wherein the nano-metal rod major or minor axis in the super-structured surface is at 45 ° to the azimuth angle of the first polarization direction.
9. The wavelength tunable vortex fiber laser of claim 1 wherein the nano-metal rod arrays in the super-structured surface are arranged in 8 groups, the 8 groups of nano-metal rod arrays having four long and wide size settings, wherein the 4 groups of nano-metal rod arrays are 90 ° out of azimuth from the other 4 groups of nano-metal rod arrays, and the cross-polarization phase shift between adjacent groups of nano-metal rod array elements is increased by pi/4.
10. The wavelength tunable vortex fiber laser of claim 1 wherein the pump source is 976nm continuous light; the working wavelength of the wavelength division multiplexer is 976/1030nm or 976/1550nm.
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| CN117748276A (en) * | 2024-02-05 | 2024-03-22 | 深圳大学 | Optical fiber laser for medium infrared wide spectrum tunable narrow linewidth |
| CN117748276B (en) * | 2024-02-05 | 2024-05-10 | 深圳大学 | Optical fiber laser for medium infrared wide spectrum tunable narrow linewidth |
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