CN111884027B - Multi-wavelength fiber laser based on two-dimensional active pi phase shift fiber grating - Google Patents

Multi-wavelength fiber laser based on two-dimensional active pi phase shift fiber grating Download PDF

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CN111884027B
CN111884027B CN202010737343.1A CN202010737343A CN111884027B CN 111884027 B CN111884027 B CN 111884027B CN 202010737343 A CN202010737343 A CN 202010737343A CN 111884027 B CN111884027 B CN 111884027B
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fiber
phase shift
active
dimensional
laser
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CN111884027A (en
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王俊
徐攀
朱靖
胡正良
车宗伦
徐志明
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National University of Defense Technology
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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
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/02123—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating
    • 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
    • H01S3/08086—Multiple-wavelength emission

Abstract

The invention discloses a multi-wavelength fiber laser based on a two-dimensional active pi phase shift fiber grating, which comprises a pumping source, a wavelength division multiplexer and a resonant cavity which are sequentially connected in series, wherein the resonant cavity is formed by the two-dimensional active pi phase shift fiber grating, the two-dimensional active pi phase shift fiber grating comprises a fiber core, the fiber core comprises more than two sub-active pi phase shift fiber gratings, each sub-active pi phase shift fiber grating is parallel to the central axis of the fiber core, and the sub-active pi phase shift fiber gratings are not overlapped and collinear. The invention has the advantages of compact structure, small volume and low cost.

Description

Multi-wavelength fiber laser based on two-dimensional active pi phase shift fiber grating
Technical Field
The invention relates to the field of fiber lasers, in particular to a multi-wavelength fiber laser based on a two-dimensional active pi phase shift fiber grating.
Background
In the field of fiber optic communication and fiber optic sensing, wavelength division multiplexing is one of the most common multiplexing modes of a system, and the multiplexing mode requires a system light source to have a plurality of laser wavelength outputs. Generally, the most straightforward way to achieve a multi-wavelength laser output is to simply combine several single-wavelength lasers having different wavelengths. However, as the communication capacity of the system increases, the network size becomes larger, and simply increasing the number of single-wavelength lasers leads to a series of problems such as higher system cost, higher power consumption, and higher complexity. Therefore, there is a trend toward the development of compact multi-wavelength lasers having a simple structure.
Among many lasers, the fiber laser has the advantages of good beam quality, high coupling efficiency, easy integration, small volume, convenient output control, small insertion loss and the like, and is particularly suitable for fiber communication and fiber sensing networks. The working principle of the multi-wavelength fiber laser is related to the implementation scheme thereof, and the following implementation modes are mainly adopted:
1) realizing multi-wavelength output by constructing a multi-laser resonant cavity;
2) realizing multi-wavelength output by using an intra-cavity comb filter device;
3) the multi-wavelength selective output is realized by using a selective feedback device, such as a multi-level grating series-parallel connection;
4) and realizing multi-wavelength output by using the difference of the conduction modes and the change of the polarization state in the optical fiber.
In general, a gain medium is required to realize laser gain over a wide frequency band, which is the basis for realizing a multi-wavelength fiber laser over a wide frequency band; and secondly, different devices are adopted to realize selective oscillation lasing of multiple wavelengths, which is the key for realizing the multi-wavelength fiber laser.
At present, most of reported multi-wavelength fiber lasers are ring cavity lasers based on erbium doped fibers. Due to the fact that the ring cavity is long, the longitudinal mode interval of the laser is small, the phenomena of multi-longitudinal mode oscillation output and mode jump often occur, and the performance of the laser is seriously affected. In order to suppress mode hopping, ring cavity fiber lasers are generally complex in structure and costly.
Disclosure of Invention
The technical problem to be solved by the invention is to overcome the defects of the prior art and provide a multi-wavelength fiber laser based on a two-dimensional active pi-phase shift fiber grating, which has the advantages of compact structure, small volume and low cost.
In order to solve the technical problems, the invention adopts the following technical scheme:
a multi-wavelength fiber laser based on a two-dimensional active pi phase shift fiber grating comprises a pumping source, a wavelength division multiplexer and a resonant cavity which are sequentially connected in series, wherein the resonant cavity is formed by the two-dimensional active pi phase shift fiber grating, the two-dimensional active pi phase shift fiber grating comprises a fiber core, the fiber core comprises more than two sub-active pi phase shift fiber gratings, each sub-active pi phase shift fiber grating is parallel to the central axis of the fiber core, and the sub-active pi phase shift fiber gratings are not overlapped and not collinear.
And the refractive indexes of the sub-active pi-phase shift fiber gratings are different.
The longitudinal mode frequency of the sub-active pi phase shift fiber grating is as follows:
Figure BDA0002605587040000021
wherein n iskDenotes the refractive index of the kth sub-active pi-phase shift fiber grating, q denotes the number of longitudinal mode modes, c denotes the speed of light in vacuum, LkAnd the length of a resonant cavity formed by the kth sub-active pi-phase shift fiber grating is larger than or equal to 1 and smaller than or equal to m, wherein k is a positive integer, and m is the number of the sub-active pi-phase shift fiber gratings.
The grating area of the two-dimensional active pi phase shift fiber grating is rare earth ion doped fiber.
The rare earth ion doped fiber is an erbium-doped fiber.
The sub-active pi phase shift fiber grating is integrated in the same section of grating area of the fiber core.
The multi-wavelength fiber laser also comprises an isolator and a laser output port, wherein the isolator is connected between the wavelength division multiplexer and the laser output port.
The resonant cavity is a linear cavity.
The writing of the two-dimensional active pi phase shift fiber grating comprises the following steps:
s1, preparing a two-dimensional phase mask plate: manufacturing a two-dimensional phase mask plate with a two-dimensional coding structure, wherein the two-dimensional phase mask plate is sequentially provided with more than two periodic structures at intervals along the width direction, and the periodic structures correspond to the sub-active pi phase shift fiber gratings one by one;
s2, building an engraving device: removing a coating layer on the surface of an active rare earth ion doped optical fiber to be inscribed, clamping the optical fiber by using a clamp system, adjusting the levelness and the verticality of the optical fiber by the clamp system through a displacement system, respectively installing two-dimensional phase mask plates and a substrate coated with fluorescent substances on the surface on two sides of the optical fiber, wherein the two-dimensional phase mask plates are tightly attached to the optical fiber and arranged in a direction close to one side of an excimer laser, and the substrate is arranged at a distance from the optical fiber and in a direction far away from one side of the excimer laser;
s3, alignment of writing spot and core: opening an excimer laser to output laser, irradiating the laser on a two-dimensional phase mask plate through adjustment of a shaping light path to form writing light spots, forming far-field diffraction stripes on a substrate, observing the far-field diffraction stripes, and adjusting a displacement system to enable the axis of an optical fiber to be parallel to the spacing lines of the two-dimensional phase mask plate when the axis of the optical fiber and the spacing lines of the two-dimensional phase mask plate have an included angle alpha;
s4, opening a wide-spectrum light source and a spectrometer, carrying out online monitoring on the transmission spectrum of the optical fiber, detecting the transmission spectrum output by the spectrometer, finely adjusting the horizontal angle and the upper and lower positions of the optical fiber in real time according to the transmission spectrum, and changing the distribution of each periodic structure on the fiber core until each periodic structure is uniformly distributed on the fiber core;
and S5, monitoring the change of the transmission spectrum, controlling the writing time according to the preset requirement, and writing the optical fiber to obtain the two-dimensional active pi-phase shift fiber grating.
In step S4, when the depression descending degrees corresponding to the wavelengths in the transmission spectrum are not equal and the descending degree of the previous depression is greater than the descending degree of the next depression, the optical fiber is fine-tuned downward until the depression descending degrees corresponding to different wavelengths in the transmission spectrum are equal or symmetrical, otherwise, the optical fiber is fine-tuned upward until the depression descending degrees corresponding to different wavelengths in the transmission spectrum are equal or symmetrical.
In the step S5, during the writing, the change of the transmission spectrum is monitored according to the preset sub-grating parameters of the optical fiber, and the writing time is controlled to obtain the required two-dimensional active pi-phase shift fiber grating.
In step S3, the shaping optical path includes a diaphragm, a first cylindrical lens, a slit, a second cylindrical lens, and a third cylindrical lens, which are sequentially disposed, the diaphragm changes the laser into a rectangular spot, the first cylindrical lens changes the rectangular spot into a linear focusing spot, the slit focuses the spot and performs spatial filtering, the second cylindrical lens changes the focused spot into a rectangular uniform spot, and the third cylindrical lens focuses the uniform spot.
Compared with the prior art, the invention has the advantages that:
the invention provides a multi-wavelength fiber laser based on a two-dimensional active pi phase shift fiber grating, which adopts the two-dimensional active pi phase shift fiber grating as a resonant cavity, wherein the two-dimensional active pi phase shift fiber grating comprises a fiber core, the fiber core comprises more than two sub-active pi phase shift fiber gratings, and the sub-active pi phase shift fiber gratings are parallel to the central axis of the fiber core and are not overlapped with each other. When the pumping light of the pumping source is injected into a resonant cavity formed by the two-dimensional active pi phase shift fiber grating, different sub-active pi phase shift fiber gratings generate single longitudinal mode laser oscillation with different wavelengths to realize multi-wavelength laser output.
Compared with the conventional active pi phase shift fiber grating based on fiber rough machining, which only forms pi phase shift refractive index modulation in the axial direction of a fiber core, the two-dimensional active pi phase shift fiber grating provided by the invention is more finely machined for an active doped fiber. The two-dimensional phase mask plate with a plurality of periodic structures is used for enabling the writing light spot to have a two-bit coding structure. When the writing light spot acts on the fiber core of the optical fiber to be written, on the same fiber core scale, not only the refractive index modulation is carried out in the axial direction, but also in the radial direction, a plurality of parallel sub-grating structures are formed, two dimensions of the axial direction and the radial direction are involved, and the optical fiber is a three-dimensional structure based on the essence of the optical fiber, so that the refractive index modulation of a three-dimensional space is realized.
When the two-dimensional active pi phase shift Fiber grating is engraved, the two-dimensional active pi phase shift Fiber grating is formed by one-time photoetching based on space coding near field diffraction, so that a foundation is laid for an In-Fiber small-scale complex optical system, the engraving efficiency of the two-dimensional active pi phase shift Fiber grating can be improved, the manufacturing cost of the grating is reduced, and batch production and application are expected to be realized. During writing, based on far-field Moire diffraction fringe observation and on-line spectrum monitoring, the quality of the two-dimensional active pi-phase shift fiber grating can be effectively improved, and the manufacturing difficulty of the two-dimensional active pi-phase shift fiber grating is reduced.
Drawings
Fig. 1 is a schematic structural view of the present invention.
FIG. 2 is a schematic diagram of the structure of a two-dimensional active π phase shifting fiber grating according to the present invention.
Fig. 3 is a schematic structural diagram of a two-dimensional active pi-phase shift fiber grating according to an embodiment of the present invention.
FIG. 4 is a schematic diagram of a two-dimensional active π phase-shift fiber grating writing system according to the present invention.
FIG. 5 is a schematic diagram of the alignment of the core and diffracted light of an optical fiber according to the present invention.
FIG. 6 is a schematic diagram of a two-dimensional active π phase shift fiber grating writing platform according to the present invention.
FIG. 7 is a diagram showing the relationship between the position of the sub-gate region and the transmission spectrum.
The reference numerals in the figures denote: 1. a pump source; 2. a wavelength division multiplexer; 3. a two-dimensional active pi-phase shift fiber grating; 31. a fiber core; 32. a cladding layer; 311. a sub-active pi-phase shift fiber grating; 4. an isolator; 5. a laser output port; 6. an excimer laser; 7. a diaphragm; 8. a first cylindrical lens; 9. a slit; 10. a second cylindrical lens; 11. a third cylindrical lens; 12. an optical platform; 13. a two-dimensional phase mask plate; 131. a periodic structure; 132. spacing lines; 14. a substrate; 15. a spectrometer; 16. a broad spectrum light source; 17. an optical fiber displacement adjusting module; 171. a clamp; 172. a vertical displacement adjustment member; 173. a left-right displacement adjusting member; 174. a front and rear displacement adjustment member; 18. dry plate clamping; 19. an optical window; 20. and a mask plate displacement adjusting module.
Detailed Description
The invention will be described in further detail below with reference to the drawings and specific examples. Unless otherwise specified, the instruments or materials employed in the present invention are commercially available.
As shown in fig. 1, the multi-wavelength fiber laser based on the two-dimensional active pi phase shift fiber grating of the present invention includes a pumping source 1, a wavelength division multiplexer 2 and a resonant cavity connected in series in sequence, the resonant cavity is composed of a two-dimensional active pi phase shift fiber grating 3, the two-dimensional active pi phase shift fiber grating 3 includes a fiber core 31 and a cladding 32 coated outside the fiber core 31, the fiber core 31 includes more than two sub-active pi phase shift fiber gratings 311, each sub-active pi phase shift fiber grating 311 is parallel to a central axis of the fiber core 31, and each sub-active pi phase shift fiber grating 311 is not overlapped and not collinear. When the pump light of the pump source 1 is injected into the resonant cavity formed by the two-dimensional active pi phase shift fiber grating 3, different sub-active pi phase shift fiber gratings 311 generate single longitudinal mode laser oscillation with different wavelengths, and multi-wavelength laser output is realized.
Unlike a conventional single-wavelength linear cavity fiber laser, as shown in fig. 1, the main body of the resonant cavity of the laser is a two-dimensional active pi-phase shift fiber grating 3 (as shown in fig. 2), and the two-dimensional active pi-phase shift fiber grating 3 is an active pi-phase shift fiber grating having complex refractive index modulation in both the radial and axial directions of the fiber. The refractive index of each sub-active pi-phase shift fiber grating 311 has different periodic distribution structures along the axial direction, so that the fiber gratings have different laser output spectral characteristics, and the multiple sub-active pi-phase shift fiber gratings 311 have multi-wavelength laser output spectral characteristics in a comprehensive manner.
The grating area of the two-dimensional active pi-phase shift fiber grating 3 is a rare earth ion doped fiber, generally an erbium doped fiber.
The sub-active pi-phase shift fiber grating 311 is integrated in the same grating region of the fiber core 31, and the fiber core 31 includes two sub-active pi-phase shift fiber gratings 311 (as shown in fig. 3) in this embodiment.
The multi-wavelength fiber laser further comprises an isolator 4 and a laser output port 5, wherein the isolator 4 is connected between the wavelength division multiplexer 2 and the laser output port 5. For fiber optic communication and fiber sensing applications, the laser output wavelength is in the C-band, and therefore the center wavelength of the pump source 1 is typically 980 nm. 980nm pump light output by the pump source 1 is injected into a resonant cavity formed by the two-dimensional active pi-phase shift fiber grating 3 through the 980/1550nm wavelength division multiplexer 2, and the amplified laser light is generated in the resonant cavity, so that the generated C-band multi-wavelength laser light is output from a 1550nm port (i.e. a laser output port 5, in this embodiment, a jumper head) of the wavelength division multiplexer 2, and the isolator 4 of the laser output port 5 is used for preventing reflected light from influencing the resonant cavity.
The resonant cavity is a linear cavity.
In this embodiment, taking a three-wavelength laser as an example, the two-dimensional active pi-phase shift fiber grating 3 is implemented by writing three sub-active pi-phase shift fiber gratings 311 in parallel in the fiber core 31 of a section of erbium-doped fiber, and the central wavelengths corresponding to the three sub-active pi-phase shift fiber gratings 311 are λ1、λ2And λ3Wherein λ is1≠λ2≠λ3. In other embodiments, the number of sub-active π -phase shifted fiber gratings 311 is greater than 2, as shown in FIG. 3, the number of sub-active π -phase shifted fiber gratings 311 is 2.
The conventional active pi phase shift fiber grating only has a single grating structure and corresponds to a unique central wavelength, so that single-wavelength fiber laser is output, and the longitudinal mode frequency of the light wave is as follows:
Figure BDA0002605587040000051
wherein n represents the effective refractive index of the medium in the laser resonant cavity, q represents the mode number of the longitudinal mode, and takes the value of an integer of 0,1,2 …, etc., c represents the light speed in vacuum, and L is the cavity length of the resonant cavity. The spacing between adjacent longitudinal modes in the cavity is
Figure BDA0002605587040000052
Linear cavity fiber lasers typically have a cavity length on the order of centimeters and produce laser light that is typically a single longitudinal mode laser, i.e., a single wavelength laser.
In the present invention, the resonant cavity is a two-dimensional active pi-phase shift fiber grating 3. In the fiber core 31 area of the two-dimensional active pi phase shift fiber grating 3, a plurality of sub-active pi phase shift fiber gratings 311 are distributed in parallel up and down along the axial direction of the fiber core 31, and each sub-active pi phase shift fiber grating 311The active pi-phase shift fiber grating 311 has different grating structure parameters, which mainly means that each sub-active pi-phase shift fiber grating 311 has different effective refractive index n, which is recorded as nkAnd k is 1,2,3 …. Thus, different sub-active π phase shifted fiber gratings 311 correspond to different longitudinal mode frequencies, which can be expressed as
Figure BDA0002605587040000053
Wherein n iskDenotes the refractive index of the kth sub-active pi-phase shift fiber grating 311, q denotes the number of longitudinal mode modes, c denotes the speed of light in vacuum, LkFor the cavity length of the resonant cavity formed by the kth sub-active pi-phase shift fiber grating 311, k is greater than or equal to 1 and less than or equal to m, k is a positive integer, and m is the number of the sub-active pi-phase shift fiber gratings 311.
The working principle of the invention is as follows:
adjusting the power of the pump light of the pump source 1 to enable the power output of the pump light to exceed the threshold power capable of generating laser oscillation, wherein the power output of the pump light is generally 100 mW;
when pump light is injected into a resonant cavity formed by the two-dimensional active pi phase shift fiber grating 3, different sub-active pi phase shift fiber gratings 311 generate single longitudinal mode laser oscillation with different wavelengths, and multi-wavelength single longitudinal mode fiber laser is output;
the output condition of the multi-wavelength laser can be monitored through the spectrometer.
The invention relates to a writing device for writing a two-dimensional active pi-phase shift fiber grating 3, which comprises an optical platform 12, an excimer laser 6, a shaping light path, a clamp system, a displacement system, a two-dimensional phase mask plate 13, a broad spectrum light source 16, a spectrometer 15 and a substrate 14 coated with a bleaching agent on the surface; the clamp system (i.e. the clamp 171 of the optical fiber displacement adjusting module 17) is used for clamping the optical fiber and is positioned on the optical platform 12, and the displacement system (i.e. the vertical displacement adjusting part 172, the left and right displacement adjusting parts 173 and the front and back displacement adjusting part 174 of the optical fiber displacement adjusting module 17) is used for adjusting the levelness and the verticality of the optical fiber; the two-dimensional phase mask plate 13 is arranged close to the optical fiber and parallel to the optical fiber, the two-dimensional phase mask plate 13 is sequentially provided with N periodic structures 131 at intervals along the width direction, the periodic structures 131 correspond to the sub-active pi phase shift optical fiber gratings 311 one by one, and N is a positive integer greater than or equal to 2; the wide-spectrum light source 16 is connected to one end of the optical fiber, and the spectrometer 15 is connected to the other end of the optical fiber;
laser spots output by the excimer laser 6 are irradiated on the two-dimensional phase mask plate 13 after being adjusted by the shaping light path, far field diffraction fringes are formed on the substrate 14, the far field diffraction fringes are observed, and the optical fiber levelness and the verticality are adjusted by the optical fiber displacement adjusting module 17 and the mask plate displacement adjusting module 20.
The two-dimensional phase mask 13 of this embodiment is different from a conventional one-dimensional phase mask (which has only one periodic structure), the two-dimensional phase mask 13 of this embodiment carries two-dimensional encoded information, the amount of information depends on the number and form of the periodic structures 131, and the number of the periodic structures 131 depends on the number of the sub-active pi-phase shift fiber gratings 311.
The invention uses the writing principle of the two-dimensional phase mask plate 13 as follows:
light spots generated by the excimer laser 6 pass through the two-dimensional phase mask plate 13 to generate near-field diffraction interference fringes with two-dimensional space coding information, and if an optical fiber to be written is close to the two-dimensional phase mask plate 13, the interference fringes form permanent refractive index periodic disturbance in the fiber core 31 with photosensitivity. By selecting proper etching depth of the two-dimensional phase mask plate 13, 0-order diffraction light can be restrained to 5% of the light intensity of incident light beams, and +/-1-order diffraction light energy reaches about 40% of the incident light energy, so that if the period of the two-dimensional phase mask plate 13 is lambdamask(fringe period), the period of the grating is ΛmaskAnd/2, independent of the wavelength of the light source. By replacing different two-dimensional phase mask plates 13, the two-dimensional active pi phase shift fiber grating 3 with different spectral characteristics can be realized.
Two key points for writing the two-dimensional active pi phase shift fiber grating 3 by adopting the two-dimensional phase mask plate 13 are as follows: the two-dimensional coding diffraction light spot is generated, and the writing light spot is aligned with the fiber core 31. The ultraviolet light facula that excimer laser 6 produced will produce single slit diffraction and multislot interference after passing through two-dimensional phase mask plate 13 that has two-dimensional code structure, if incident light covers the whole district that encodes district of two-dimensional phase mask plate 13, the interference fringe that produces will carry the whole coding information of two-dimensional phase mask plate 13, the interference fringe has many parallel distribution of lines structure promptly, such fringe facula has certain spatial dimension, the facula only aligns with fibre core 31 is accurate, can carry out meticulous microfabrication to fibre core 31 at wavelength scale.
When in writing, the two key points can be effectively solved based on far-field Moire diffraction fringe observation. The basic principle is as follows:
as shown in fig. 4, after being expanded, collimated and focused, ultraviolet light spots for grating writing sequentially pass through the two-dimensional phase mask plate 13 of the two-dimensional space code and the single-core optical fiber to be written closely attached to the rear surface of the two-dimensional phase mask plate 13, and then fresnel near-field diffraction occurs. The single-core optical fiber to be etched has an extended fringe perpendicular to the axial direction of the optical fiber in the far field, and at the same time, the gap (marked as the gap line 132) between the periodic structures 131 of the two-dimensional phase mask 13 is approximately in the order of μm, and can be approximated to a single slit meeting the diffraction condition, and the periodic structures 131 also have an extended fringe perpendicular to the single slit in the far field, and the extended fringe (i.e., the diffraction fringe pattern) is irradiated on the substrate 14 (in this embodiment, common white paper) containing the bleaching agent, and the diffraction fringe pattern can be seen by human eyes by exciting visible green fluorescence.
In actual practice, the spacing lines 132 between the optical fibers and the periodic structure 131, which is approximately a single slit, may not be in a parallel state, as shown in fig. 5 (a). When a slight angle α exists between the fiber axis relative to the spacer lines 132, far field moire diffraction fringes observed on the substrate 14 are shown in fig. 5 (c). As the angle α between the spacing lines 132 between the fibers and the periodic structure 131 (i.e., the two-dimensional phase mask 13) changes, the spacing (or period) of the far-field moire diffraction fringes changes. The horizontal angle and the up and down position of the fiber can be changed by adjusting the fine displacement stage shown in fig. 6 to finally make the angle α zero as shown in fig. 5(b), and the far field moire diffraction fringe is shown in fig. 5(d), which shows that the spacing lines 132 between the fiber and the periodic structure 131 are substantially parallel, i.e. the alignment of the writing spot with the fiber core 31 is achieved. Meanwhile, by comparing the longitudinal position between the fiber diffraction principal maximum and the spacing line 132 of the two-dimensional phase mask plate 13, the required writing light spot can be focused by the third cylindrical lens 11 and then fall on the central fiber core 31 of the fiber.
The two-dimensional active pi phase shift fiber grating 3 of the present embodiment includes three steps: preparing a two-dimensional phase mask plate 13, building a writing device and monitoring writing of fiber bragg gratings.
Firstly, the preparation process of the two-dimensional phase mask 13 of this embodiment is:
1) decomposing argon into argon ions by using a glow discharge principle;
2) the acceleration of the argon ions by the anode electric field physically bombards the sample surface, causing the photoresist to be dislodged or removed from the surface, thereby exposing the matrix material.
Second, the construction of the inscribing system and the platform of the embodiment
The method comprises the following steps of building a two-dimensional active pi phase shift fiber grating writing system based on an ultraviolet lithography phase mask method, adjusting a light path, and generating diffracted light with a space coding structure, wherein the specific implementation process comprises the following steps:
1) according to the system schematic diagram shown in fig. 4, a light path is established on an optical platform 12, and the light path comprises a diaphragm 7, a first cylindrical lens 8, a slit 9, a second cylindrical lens 10 and a third cylindrical lens 11;
2) opening the excimer laser 6 to output 248nm ultraviolet laser;
3) adjusting the diaphragm 7 to enable ultraviolet light to become a rectangular light spot after passing through the diaphragm 7;
4) the rectangular light spot passes through the first cylindrical lens 8 to obtain a linear focusing light beam, the position of the slit 9 is adjusted to the focus of the first cylindrical lens 8, and spatial filtering is carried out on the focusing light spot;
5) adjusting the position of the second cylindrical lens 10 to enable the distance between the slit 9 and the second cylindrical lens 10 to be one time of the focal length of the second cylindrical lens 10, so that rectangular and uniform light spots after shaping are obtained;
6) placing the two-dimensional phase mask plate 13 at a proper position behind the third cylindrical lens 11, and clamping the two-dimensional phase mask plate 13 by using a dry plate clamp 18 to keep the two-dimensional phase mask plate 13 vertical to the optical platform 12;
7) the rectangular uniform light spot collimated by the second cylindrical lens 10 is focused by the third cylindrical lens 11 and then irradiates on the two-dimensional phase mask plate 13, and +/-1-order diffraction is generated on the rear surface of the two-dimensional phase mask plate 13 to form interference fringes.
Third, monitoring of fiber grating
The preparation of the two-dimensional active pi phase shift fiber grating 3 and the on-line monitoring of the writing quality are started, and the specific implementation process is as follows:
1) removing a coating layer of the active rare earth ion doped optical fiber to be etched, and enabling the exposed length of the bare fiber to be slightly larger than the length of the two-dimensional phase mask plate 13 to finish the preparation of the optical fiber to be etched;
2) the method comprises the following steps of clamping parts, containing coating layers, of two ends of an optical fiber to be etched on two optical fiber displacement adjusting modules 17 on two sides of a two-dimensional phase mask plate 13 respectively, wherein the optical fiber displacement adjusting modules 17 are located on an optical platform 12, the optical fiber displacement adjusting modules 17 are used for adjusting the position of the optical fiber, and the optical fiber displacement adjusting modules 17 are six-degree-of-freedom precise optical fiber alignment platforms; the fiber displacement adjusting module 17 is provided with a clamp 171 for clamping the optical fiber, a vertical displacement adjusting member 172 for adjusting the vertical displacement of the optical fiber, a left-right displacement adjusting member 173 for adjusting the left-right displacement of the optical fiber, and a front-back displacement adjusting member 174 for adjusting the front-back displacement of the optical fiber. The two-dimensional phase mask plate 13 and the optical window 19 are clamped through the dry plate clamp 18, the optical fiber is located between the two-dimensional phase mask plate 13 and the optical window 19, and a mask plate displacement adjusting module 20 used for adjusting the position of the mask plate is arranged below the dry plate clamp 18. In this embodiment, the mask plate displacement adjusting module 20 is a six-degree-of-freedom precision optical fiber collimating stage;
3) finely adjusting an optical fiber displacement adjusting module 17, and observing the relative distance between the bare fiber part of the optical fiber to be inscribed and the two-dimensional phase mask plate 13 by using a movable visible electron microscope to ensure that the distances at all positions are equal;
4) placing common white paper containing bleaching agent on a position more than 1 meter away from the optical fiber and perpendicular to the ultraviolet light beam plane as a substrate 14, and adjusting the laser parameters of the excimer laser 6 to ensure that the repetition frequency is lower than 1Hz and the laser power is reduced to the extent that obvious blue fluorescence can be observed on the white paper; in other embodiments, the substrate 14 can achieve the same or similar technical effects when located in the far-field region of the shaped light spot;
5) finely adjusting the optical fiber displacement adjusting module 17 and the mask plate displacement adjusting module 20 until diffraction fringes of the optical fiber on the substrate 14 are parallel to diffraction fringes of a sub-grid region (a periodic structure 131 of the two-dimensional phase mask plate 13) and spacing lines 132;
6) the optical fiber is moved integrally in the vertical direction through the fine adjustment optical fiber displacement adjusting module 17, and diffraction fringes are observed, so that the required sub-grating area is superposed with the main maximum position of the diffraction fringes of the optical fiber to be inscribed;
7) adjusting parameters such as pulse power, repetition frequency and the like of the excimer laser 6 to meet the grating writing requirements, and starting ultraviolet light if the pulse power is set to be 120mJ and the repetition frequency is set to be 10 Hz;
8) connecting a wide-spectrum light source 16 and a spectrometer 15 to two ends of an optical fiber to be written according to the graph shown in FIG. 4, observing the transmission spectrum of the optical fiber through the spectrometer 15, judging the actual position of the sub-grating region at the fiber core 31 according to the schematic diagram of the relation between the position of the sub-grating region and the transmission spectrum, finely adjusting the optical fiber in the vertical direction through a fine-adjustment optical fiber displacement adjusting module 17, and gradually correcting the transmission spectrum;
9) according to the needed grating parameters, the required two-dimensional active pi phase shift fiber grating 3 is obtained by monitoring the transmission spectrum to control the writing time, and if the writing time is 1 minute, the reflectivity of the fiber grating can reach 20dB (the longer the writing time is, the higher the grating reflectivity is).
During writing, the spectrometer 15 is used to monitor the spectral output in real time, distinguish the relative proportion of energy at the transmission or reflection wavelength corresponding to each sub-grating region, and finely adjust the horizontal angle and the up-down position of the optical fiber in real time, so as to change the uniformity of the distribution of each sub-grating region on the fiber core 31.
In this example, the positional relationship between the brightest white region in the middle of the backgrounds of fig. 7(a), (c), and (e) and the respective sub-grating structures is considered, and fig. 7(a) shows the position of the optical fiber at λ with respect to the two-dimensional phase mask 131、λ2In the center of the sub-gate region (periodic structure 131), λ1、λ2The sub-gratings uniformly fall in the backgroundWhite area between, lambda3The sub-grating does not fall in the middle white region, and the transmission spectrum in fig. 7(b) shows two pits corresponding to the wavelengths and the degree of the fall is equal (each pit has a small tip protrusion, which is consistent with the characteristic of pi phase shift); in FIG. 7(c) the fiber is positioned close to λ1Sub-gate region, λ1The sub-gratings falling completely in the white region, λ2Only a part falls on the white area, so λ1The transmission peak of (a) is stronger, and the wavelength of the transmission spectrum light is shown as lambda in FIG. 7(d)2The optical power drop at the corresponding recess is reduced. FIG. 7(e) shows the fiber at λ2In the center of the sub-grid region, λ2With the sub-grating in the middle of the white background region, λ1And λ3Only a part falls in the white region, λ2Maximum of transmission peak, λ1And λ3Smaller, three corresponding transmission dips appear in FIG. 7(f), where the transmission trims the spectral wavelength λ2And the optical power is reduced to the maximum extent at the corresponding recess in the center.
And the online monitoring of the writing quality of the two-dimensional active pi-phase shift fiber grating 3 is realized through the observation of far-field Moire diffraction fringes and the online spectrum monitoring and discrimination.
Although the present invention has been described with reference to the preferred embodiments, it is not intended to be limited thereto. Those skilled in the art can make numerous possible variations and modifications to the present invention, or modify equivalent embodiments to equivalent variations, without departing from the scope of the invention, using the teachings disclosed above. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical spirit of the present invention should fall within the protection scope of the technical scheme of the present invention, unless the technical spirit of the present invention departs from the content of the technical scheme of the present invention.

Claims (9)

1. The utility model provides a multi-wavelength fiber laser based on two-dimentional active pi phase shift fiber grating, includes pump source (1), wavelength division multiplexer (2) and the resonant cavity that connects gradually, its characterized in that: the resonant cavity is formed by a two-dimensional active pi phase shift fiber grating (3), the two-dimensional active pi phase shift fiber grating (3) comprises a fiber core (31), the fiber core (31) comprises more than two sub-active pi phase shift fiber gratings (311), each sub-active pi phase shift fiber grating (311) is parallel to the central axis of the fiber core (31), and the sub-active pi phase shift fiber gratings (311) are not overlapped and not collinear.
2. The multi-wavelength fiber laser of claim 1, wherein: the refractive indexes of the sub-active pi-phase shift fiber gratings (311) are different.
3. The multi-wavelength fiber laser of claim 2, wherein: the longitudinal mode frequency of the sub-active pi phase shift fiber grating (311) is as follows:
Figure FDA0002605587030000011
wherein n iskDenotes the refractive index of the kth sub-active pi-phase shift fiber grating (311), q denotes the number of longitudinal mode modes, c denotes the speed of light in vacuum, LkThe length of a resonant cavity formed by the kth sub-active pi-phase shift fiber grating (311) is larger than or equal to 1 and smaller than or equal to m, k is a positive integer, and m is the number of the sub-active pi-phase shift fiber gratings (311).
4. The multi-wavelength fiber laser of claim 2, wherein: the grating area of the two-dimensional active pi phase shift fiber grating (3) is a rare earth ion doped fiber.
5. The multi-wavelength fiber laser of claim 2, wherein: the sub-active pi phase shift fiber grating (311) is integrated in the same section of grating region of the fiber core (31).
6. The multi-wavelength fiber laser according to any one of claims 1 to 5, characterized in that: the multi-wavelength fiber laser further comprises an isolator (4) and a laser output port (5), wherein the isolator (4) is connected between the wavelength division multiplexer (2) and the laser output port (5).
7. The multi-wavelength fiber laser according to any one of claims 1 to 5, characterized in that: the resonant cavity is a linear cavity.
8. The multi-wavelength fiber laser according to any one of claims 1 to 5, characterized in that: the two-dimensional active pi phase shift fiber grating (3) writing method comprises the following steps:
s1, preparing a two-dimensional phase mask plate (13): manufacturing a two-dimensional phase mask plate (13) with a two-dimensional coding structure, wherein the two-dimensional phase mask plate (13) is sequentially provided with more than two periodic structures (131) at intervals along the width direction, and the periodic structures (131) correspond to the sub-active pi-phase shift fiber gratings (311) one by one;
s2, building an engraving device: removing a coating layer on the surface of an active rare earth ion doped optical fiber to be inscribed, clamping the optical fiber by using a clamp system, adjusting the levelness and the verticality of the optical fiber by the clamp system through a displacement system, respectively installing two-dimensional phase mask plates (13) and a substrate (14) coated with fluorescent substances on the surface on two sides of the optical fiber, wherein the two-dimensional phase mask plates (13) are tightly attached to the optical fiber and arranged in a direction close to one side of an excimer laser (6), and the substrate (14) is arranged at intervals with the optical fiber and in a direction far away from one side of the excimer laser (6);
s3 alignment of the writing spot and the core (31): opening an excimer laser (6) to output laser, irradiating the laser on a two-dimensional phase mask plate (13) through adjustment of a shaping light path to form a writing light spot, forming a far-field diffraction stripe on a substrate (14), observing the far-field diffraction stripe, and adjusting a displacement system to enable the axis of an optical fiber to be parallel to a spacing line (132) of the two-dimensional phase mask plate (13) when an included angle alpha exists between the axis of the optical fiber and the spacing line (132) of the two-dimensional phase mask plate (13);
s4, turning on a wide-spectrum light source (16) and a spectrometer (15), detecting a spectrum output transmission spectrum by the spectrometer (15), finely adjusting the horizontal angle and the upper and lower positions of the optical fiber in real time according to the transmission spectrum, and changing the distribution of each periodic structure (131) on the fiber core (31) until each periodic structure (131) is uniformly distributed on the fiber core (31);
and S5, monitoring the change of the transmission spectrum, controlling the writing time according to the preset requirement, and writing the optical fiber to obtain the two-dimensional single-core fiber grating.
9. The multi-wavelength fiber laser of claim 8, wherein: in step S4, when the depression descending degrees corresponding to the wavelengths in the transmission spectrum are not equal and the descending degree of the previous depression is greater than the descending degree of the next depression, the optical fiber is fine-tuned downward until the depression descending degrees corresponding to different wavelengths in the transmission spectrum are equal or symmetrical, otherwise, the optical fiber is fine-tuned upward until the depression descending degrees corresponding to different wavelengths in the transmission spectrum are equal or symmetrical.
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