CN115685448B - Wavelength division multiplexer, design method and manufacturing method thereof and fiber laser - Google Patents
Wavelength division multiplexer, design method and manufacturing method thereof and fiber laser Download PDFInfo
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Abstract
The invention relates to the technical field of optical instruments, in particular to a wavelength division multiplexer, a design method and a manufacturing method thereof and an optical fiber laser, wherein the wavelength division multiplexer comprises an input part, a coupling part and an output part which are sequentially connected, and the length of the coupling part is as followsWherein L is the cavity length of the fiber laser, GVD 1 GVD (Global wavelength division multiplexing) for unit length dispersion value of single-mode fiber for set wave band 2 The total dispersion value per unit length is set to the coupling portion diameter. By designing the length of the coupling part, the cross section diameter of the coupling part and other related parameters, the wavelength division multiplexer can provide anomalous dispersion accumulation and filter spontaneous emission light of a non-set wave band, namely noise light. The wavelength division multiplexer is connected into the optical fiber laser with the set wave band based on the neodymium-doped optical fiber, can offset or partially offset the normal dispersion in the optical fiber laser, and simultaneously filters noise light with other wave bands, thereby helping to start and stabilize mode locking.
Description
Technical Field
The present invention relates to the field of optical instruments, and in particular, to a wavelength division multiplexer, a design method and a manufacturing method thereof, and an optical fiber laser.
Background
The ultra-short pulse laser with the wave band of 920nm has good application in the aspect of two-photon fluorescence microscopic imaging in the field of neuroscience. The prior art methods for generating 920nm ultrashort pulse laser (also referred to as "mode locking") are generally as follows: a titanium sapphire tunable laser or optical parametric oscillation method is used; by utilizing the nonlinear effect of the optical fiber, the optical fiber is subjected to frequency shifting, frequency doubling and other treatments from other wave bands (such as 1040nm wave band corresponding to ytterbium-doped optical fiber, 1560nm wave band corresponding to erbium-doped optical fiber and 1.9 mu m wave band corresponding to thulium-doped optical fiber) to 920nm wave band; the seed source of the neodymium-doped optical fiber is directly utilized to generate the pulse of the wave band. The first scheme relies on a large amount of space optical devices, so that the requirements on the mechanical stability and the thermal stability of the laser are high; the second type of scheme is characterized in that the core is a frequency shifting and frequency doubling mechanism, devices such as a photonic crystal fiber and a frequency doubling crystal are very dependent, the cost is additionally increased, and the quality control of the final output light beam is still a troublesome problem.
The third scheme is that the semiconductor optical pump source with 808nm wave band provides energy. Neodymium ions doped in optical fibers (Nd 3+ ) After absorbing the pump light, the population inversion is realized, and spontaneous emission light containing 920nm wave band is generated. The stimulated radiation is formed by cyclic oscillation and amplification in the laser cavity, namely, the continuous laser with the wavelength of 920nm is formed. When there is a suitable saturable absorption mechanism (such as a semiconductor saturable absorber mirror, a polarization control component similar to a saturable absorber, a phase shift control component and the like) in the cavity, an ultra-short pulse (usually in the order of picoseconds or femtoseconds) laser with the wavelength of 920nm can be formed, namely, a mode locking state is achieved. Through pulse compression operationFor example, a grating pair or a chirped mirror is used at the output end to provide anomalous dispersion, and thus a femtosecond ultra-short pulse laser can be obtained.
However, the third category of solutions described above still faces two major inherent difficulties:
1. competition of the primary radiation band. As known from the knowledge of atomic physics and quantum mechanics, after absorbing pump photons in 808nm wave band, neodymium ions have high probability of four-level transition to generate light radiation in 1060nm-1090nm wave band; there is also a small probability of a three-level transition occurring, producing optical radiation in the 920nm band. Therefore, the main radiation peak of most neodymium-doped optical fibers is 1060-1090nm wave band, and the intensity of the main radiation peak is several times or even tens of dB higher than that of 920nm wave band. To make a 920nm band laser using a neodymium-doped fiber, one must want to suppress or filter out the spontaneous emission in the 1060-1090nm band (this band is considered as noise light) to prevent this band from forming oscillations in the laser cavity; otherwise, radiation in the 1060-1090nm band will compete and oscillate, while radiation in the 920nm band will be suppressed and cannot oscillate, i.e. 920nm laser light cannot be generated, and mode locking cannot be achieved.
2. Dispersion management problems. The quartz fiber shows stronger normal dispersion (group velocity dispersion sign is positive) in the 920nm wave band, and the quartz material made of the fiber shows normal dispersion, and the waveguide structure of the quartz fiber does not need to be specially designed and can also show normal dispersion in the wave band. In addition, the effect of self-phase modulation caused by nonlinear optical effects within the laser cavity is the same as normal dispersion, which produces an excessive co-symbol phase shift. In a typical seed source of a neodymium-doped optical fiber, since the refractive index of each longitudinal mode (i.e., the fine spectral component) in quartz has a slight difference, when the chromatic dispersion is not balanced and there is no filtering mechanism, as the light continuously oscillates in the seed source cavity, the phase difference between the light and the seed source cavity gradually increases, so that phase locking cannot be realized, and ultra-short pulses cannot be generated. Thus, trying to introduce anomalous dispersion (group velocity dispersion sign negative), providing dispersion compensation, providing opposite sign phase shifts for each longitudinal mode, and thus achieving a certain balance with self-phase modulation, is an important idea to produce 920nm ultrashort pulses. Although there are several devices on the market that can compensate for dispersion, the disadvantages are quite apparent: the transmission/blazed grating, the prism pair and the chirp reflector are all space devices, occupy a certain space, are unfavorable for miniaturization of the laser and are sensitive to mechanical vibration; the chirped fiber grating is an optical fiber device, has better resistance to mechanical vibration, but has higher design cost, and needs to be customized in a specialization way by depending on a mask, and the mask is also a consumable with higher manufacturing cost.
Disclosure of Invention
The invention provides a wavelength division multiplexer, a design method and a manufacturing method thereof and an optical fiber laser, which are used for solving one of the technical problems existing in the prior art, and realizing the effects of providing anomalous dispersion accumulation and filtering spontaneous radiation of a non-set wave band, namely noise light, through the design of the length of a coupling part, the section diameter of the coupling part and other related parameters.
The invention provides a wavelength division multiplexer, which comprises an input part, a coupling part and an output part which are connected in sequence, wherein the length of the coupling part is as followsWherein L is the cavity length of the fiber laser, GVD 1 GVD (Global wavelength division multiplexing) for unit length dispersion value of single-mode fiber for set wave band 2 The total dispersion value per unit length is set to the coupling portion diameter.
According to the wavelength division multiplexer provided by the invention, the cavity of the laser is an annular cavityWhere c is the speed of light in vacuum, n is the refractive index of light propagating in the fiber, f R Is the repetition frequency of the pulse train.
According to the wavelength division multiplexer provided by the invention, the cavity of the laser is a linear cavityc isLight velocity in vacuum, n is refractive index of light propagating in the optical fiber, f R Is the repetition frequency of the pulse train.
The wavelength division multiplexer comprises a first optical fiber and a second optical fiber, wherein the first optical fiber comprises a first coupling section, an input section and a first output section, the input section and the first output section are respectively connected with two ends of the first coupling section, the second optical fiber comprises a second coupling section and a second output section connected with one end of the second coupling section, the first coupling section and the second coupling section are opposite and are mutually arranged in parallel to form the coupling part, the input section forms the input part, and the first output section and the second output section form the output part.
The invention also provides a design method of the wavelength division multiplexer, which is applied to the wavelength division multiplexer and comprises the following steps:
obtaining the cavity length of the fiber laser according to the cavity shape of the fiber laser and the repetition frequency of the pulse sequence;
acquiring a unit length dispersion value of a single-mode fiber for a set wave band and a unit length total dispersion value when the diameter of a coupling part is the set diameter;
according to the cavity length L of the fiber laser and the dispersion value GVD of a single-mode fiber in a unit length of a set wave band 1 And a total dispersion value GVD per unit length when the diameter of the coupling part is a set diameter 2 The length l of the coupling part is obtained.
The invention also provides a method for manufacturing the wavelength division multiplexer, which is used for manufacturing the wavelength division multiplexer and comprises the following steps:
the tapered part of the first optical fiber and the tapered part of the second optical fiber are opposite and are arranged in parallel, and are separated by a set distance d;
heating and stretching the tapered part of the first optical fiber and the tapered part of the second optical fiber until the diameter of the tapered part reaches a set diameter and the length of the total tapered region reaches a set coupling part length l to form a coupling part;
the coupling portion is mechanically encapsulated.
The invention also provides an optical fiber laser based on the wavelength division multiplexer, which comprises a saturable absorption device, a first wavelength division multiplexer, a single cladding neodymium-doped optical fiber, a second wavelength division multiplexer and a laser output end which are sequentially connected, wherein the first wavelength division multiplexer and the second wavelength division multiplexer are all the wavelength division multiplexers as described above, and the set wave bands of the first wavelength division multiplexer and the second wavelength division multiplexer are different.
According to the optical fiber laser provided by the invention, the first wavelength division multiplexer is a 920/1060-1090nm wavelength division multiplexer, and the second wavelength division multiplexer is a 808/920nm wavelength division multiplexer.
According to the optical fiber laser provided by the invention, the second wavelength division multiplexer is also connected with a single-mode optical pumping source.
The wavelength division multiplexer provided by the invention is based on the principle of evanescent wave coupling and the optical fiber fusion tapering process, and can provide anomalous dispersion accumulation and filter spontaneous radiation of non-set wave bands, namely noise light, through the design of the length of the coupling part, the section diameter of the coupling part and other related parameters. Under the condition that a 920nm wave band is selected by a set wave band, the wavelength division multiplexer is connected into the optical fiber laser based on the neodymium-doped optical fiber, can offset or partially offset normal dispersion in the optical fiber laser, and simultaneously filters noise light of other wave bands, thereby helping to start and stabilize mode locking of the 920nm wave band and generating ultra-short pulse of the wave band. The ultra-short pulse laser has better application in two-photon fluorescence microscopy imaging, and can be used for high-resolution microscopy imaging in biomedical fields such as neuroscience and the like.
In addition to the technical problems, features of the constituent technical solutions and advantages brought by the technical features of the technical solutions described above, other technical features of the present invention and advantages brought by the technical features of the technical solutions will be further described with reference to the accompanying drawings or will be understood through practice of the present invention.
Drawings
In order to more clearly illustrate the embodiments of the invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, it being obvious that the drawings in the following description are only some embodiments of the invention, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
Fig. 1 is a schematic diagram of a wavelength division multiplexer according to the present invention;
FIG. 2 is a schematic diagram of a fiber laser according to the present invention;
FIG. 3 is a flow chart of a method for fabricating a wavelength division multiplexer according to the present invention;
FIG. 4 is one of the graphs of the dispersion value simulation data in the 920nm band obtained after the HI780 single mode fiber provided by the invention is simulated and tapered;
FIG. 5 is a second graph of simulation data of dispersion values in a 920nm band obtained after analog tapering of the HI780 single-mode optical fiber provided by the invention;
reference numerals:
100. an input unit; 200. a coupling section; 300. an output unit;
400. a first optical fiber; 410. a first coupling section; 420. an input section; 430. a first output section;
500. a second optical fiber; 510. a second coupling section; 520. a second output section;
610. a saturable absorber device; 620. a first wavelength division multiplexer; 630. single cladding neodymium-doped optical fiber; 640. a second wavelength division multiplexer; 650. a laser output; 660. a single mode optical pump source.
Detailed Description
Embodiments of the present invention are described in further detail below with reference to the accompanying drawings and examples. The following examples are illustrative of the invention but are not intended to limit the scope of the invention.
In the description of the embodiments of the present invention, it should be noted that the terms "center", "longitudinal", "lateral", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, are merely for convenience in describing the embodiments of the present invention and simplifying the description, and do not indicate or imply that the apparatus or elements referred to must have a specific orientation, be configured and operated in a specific orientation, and thus should not be construed as limiting the embodiments of the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.
In describing embodiments of the present invention, it should be noted that, unless explicitly stated and limited otherwise, the terms "coupled," "coupled," and "connected" should be construed broadly, and may be either a fixed connection, a removable connection, or an integral connection, for example; the mechanical connection, electrical connection, optical path connection and magnetic connection are adopted; can be directly connected or indirectly connected through an intermediate medium. The specific meaning of the above terms in embodiments of the present invention will be understood in detail by those of ordinary skill in the art.
In embodiments of the invention, unless expressly specified and limited otherwise, a first feature "up" or "down" on a second feature may be that the first and second features are in direct contact, or that the first and second features are in indirect contact via an intervening medium. Moreover, a first feature being "above," "over" and "on" a second feature may be a first feature being directly above or obliquely above the second feature, or simply indicating that the first feature is level higher than the second feature. The first feature being "under", "below" and "beneath" the second feature may be the first feature being directly under or obliquely below the second feature, or simply indicating that the first feature is less level than the second feature.
Furthermore, in the description of the embodiments of the present invention, unless otherwise indicated, the meaning of "a plurality of", "a plurality of" means two or more, and the meaning of "a plurality of", "a plurality of" means one or more ".
In the description of the present specification, a description referring to terms "one embodiment," "some embodiments," "examples," "specific examples," or "some examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the embodiments of the present invention. In this specification, schematic representations of the above terms are not necessarily directed to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, the different embodiments or examples described in this specification and the features of the different embodiments or examples may be combined and combined by those skilled in the art without contradiction.
As shown in fig. 1, the wavelength division multiplexer according to the embodiment of the present invention includes an input unit 100, a coupling unit 200, and an output unit 300, which are sequentially connected, where the length of the coupling unit 200 is as follows Wherein L is the cavity length of the fiber laser, GVD 1 GVD (Global wavelength division multiplexing) for unit length dispersion value of single-mode fiber for set wave band 2 The total dispersion value per unit length is set to the coupling diameter.
The wavelength division multiplexer of the embodiment of the invention can provide anomalous dispersion accumulation and filter spontaneous radiation of a non-set wave band, namely noise light, through the design of the length of the coupling part 200, the section diameter of the coupling part 200 and other related parameters based on the principle of evanescent wave coupling and the optical fiber fusion tapering process. Under the condition that a 920nm wave band is selected by a set wave band, the wavelength division multiplexer is connected into the optical fiber laser based on the neodymium-doped optical fiber, can offset or partially offset normal dispersion in the optical fiber laser, and simultaneously filters noise light of other wave bands, thereby helping to start and stabilize mode locking of the 920nm wave band and generating ultra-short pulse of the wave band. The ultra-short pulse laser has better application in two-photon fluorescence microscopy imaging, and can be used for high-resolution microscopy imaging in biomedical fields such as neuroscience and the like.
Assuming that 2 different wavelength bands of light are inputted from the input section 100, for example, 920nm and 1060-1090nm, by designing the length l of the coupling section 200, it is possible to make 920nm wavelength band of light output from the output section 300 at a 100% ratio, while 1060-1090nm wavelength band of light output from the output section 300 at a 100% ratio. Thus, the input unit 100 is a common terminal of 2 different wavelength bands, i.e., a wavelength division multiplexing function is achieved. When the length l of the coupling portion 200 is designed, due to the principle of evanescent wave coupling, the coupling portion has cycle repeatability, and when the length of the coupling portion 200 takes a series of values of l, l+Δl, l+2 Δl … … and the like, the wavelength division multiplexing of 2 different wave bands can be realized on the assumption that the cycle length is Δl.
The dispersion in quartz fiber is classified into material dispersion, waveguide dispersion, modal (transverse mode) dispersion, polarization mode dispersion, etc., and is often characterized by a physical quantity of group velocity dispersion (group velocity dispersion). In single mode optical fibers, since there are only 1 transverse modes and there is no significant birefringence in the fiber, the latter two factors are eliminated. The total group velocity dispersion in a single mode fiber is therefore the sum of the material dispersion and the waveguide dispersion for a certain determined optical band. The 920nm wave band shows strong positive group velocity dispersion in the quartz material, which is limited by the optical property of the quartz material and is difficult to change; however, by designing the profile, geometry, etc. of the waveguide, the magnitude of the waveguide dispersion can be adjusted to be negative, thereby balancing the effects of the material dispersion.
Tapering is a way to change the geometry of the waveguide. The cladding diameter of a common standard quartz optical fiber is 125 mu m, and can be reduced to below 10 mu m or even within 5 mu m by the prior tapering technology, and the insertion loss of a taper region is controlled. Fig. 4 and 5 are graphs of dispersion data in the 920nm band obtained by analog tapering of the HI780 single mode fiber to different cladding diameters, and it can be seen that the total group velocity dispersion is negative after tapering.
According to one embodiment of the invention, the cavity of the laser is a ring cavity Where c is the speed of light in vacuum, n is the refractive index of light propagating in the fiber, f R Is the repetition frequency of the pulse train.
In this embodiment, the HI780 single-mode optical fiber has a dispersion value of 27.8fs per unit length in the 920nm band 2 /mm. As can be seen from the simulation data of FIGS. 4 and 5, tapering the fiber to a cladding diameter of 2 μm can greatly counteract the material dispersion and reverse the total dispersion value to-34.7 fs per unit length 2 /mm. The total dispersion value per unit length is-51.2 fs when the cladding diameter is 1 mu m 2 /mm. The pulse sequence repetition frequency of the common ultra-short pulse fiber laser is 80MHz, and if the cavity structure of the fiber laser is an annular cavity, the pulse sequence repetition frequency of the annular cavity is adoptedWhere c is the speed of light in vacuum, n is the refractive index experienced by light propagating in the fiber, and L is the cavity length of the annular cavity. Available after transformation-> For quartz fiber, n=1.5 is taken to estimate, and the cavity length of the laser corresponding to the repetition frequency range is about 2.5m.
According to one embodiment of the invention, the cavity of the laser is a linear cavity c is the speed of light in vacuum, n is the refractive index of light propagating in the fiber, f R Is the repetition frequency of the pulse train.
In this embodiment, if the cavity structure of the fiber laser is a linear cavity, the pulse sequence of the linear cavity is usedIs a repetition frequency formula of (2)Available after transformation->The cavity length of the laser is about 1.25m.
Typical ultra-short pulse fiber lasers with intra-cavity dispersion compensation generally fall into the category of stretched pulse fiber lasers if classified according to the accumulated dispersion and nonlinear phase shift changes of the pulses evolving within their cavities. The fiber laser is usually of a negative dispersion stretched pulse type or a near zero dispersion stretched pulse type. If the net intra-cavity dispersion value of the broadened pulse-type fiber laser is required to be zero, the GVD is related 1 ·(L-l)+GVD 2 L=0, i.e
When the cavity structure of the fiber laser is a linear cavity, the cavity length l=1.25m, and the GVD is taken as the above value 1 ＝27.8fs 2 /mm，GVD 2 ＝-51.2fs 2 L-l=0.81 m, l=0.44 m can be solved for/mm. That is, the value of l=0.44 m is the length of the coupling portion 200 having a required diameter of 1 μm. In view of the above results, it is roughly estimated that if the length of the optical fiber in the cavity is 30-60cm or more and is tapered to a state of the diameter, the normal dispersion accumulated in other optical fibers in the 920nm band can be effectively balanced by the part of the optical fiber, so that mode locking is facilitated.
In one embodiment of the present invention, the optical fiber comprises a first optical fiber 400 and a second optical fiber 500, wherein the first optical fiber 400 comprises a first coupling section 410, an input section 420 and a first output section 430 which are respectively connected with two ends of the first coupling section 410, the second optical fiber 500 comprises a second coupling section 510 and a second output section 520 which is connected with one end of the second coupling section 510, the first coupling section 410 and the second coupling section 510 are opposite and are arranged in parallel to each other to form a coupling part 200, the input section 420 forms an input part 100, and the first output section 430 and the second output section 520 form an output part 300.
In this embodiment, the wavelength division multiplexer for the 920nm band mode-locked fiber laser is manufactured based on the fiber fusion tapering process. Within the laser, neodymium ions (Nd 3+ ) After being excited by 808nm pump light, the mixed spontaneous emission light of 920nm wave band and 1060-1090nm wave band is radiated, the mixed spontaneous emission light is input through the input section 420, and after passing through the coupling part 200 formed by the first coupling section 410 and the second coupling section 510, the light of 920nm wave band can be output from the first output section 430 of the output part 300 in a proportion of 100%, the first output section 430 is connected with other optical fiber devices in the cavity, and the light of 1060-1090nm wave band enters the second output section 520 suspended or connected with the unidirectional isolation device in a proportion of 100%, namely, is output from the second output section 520 of the output part 300. Therefore, light in the 920nm wave band can continuously oscillate in the cavity, while light in 1060-1090nm is filtered out of the cavity and cannot oscillate.
In other embodiments, light in the 920nm band may also be designed to be output from the second output segment 520 of the output 300 at a 100% ratio, while light in the 1060-1090nm band is output from the first output segment 430 of the output 300 at a 100% ratio, as is customary or desired.
In addition, by designing the length l of the coupling part 200 and possibly inserting a plurality of wavelength division multiplexers into the cavity, the wavelength division multiplexer can fully exert the filtering performance and simultaneously enable 920nm wave band light to accumulate a sufficient amount of anomalous dispersion in the coupling part 200, so that the total dispersion in the whole laser cavity can be balanced, and the laser is easier to generate mode locking pulse, namely, the wavelength division multiplexer of the invention has the dual functions of a filter and a dispersion compensation device.
The design method of the wavelength division multiplexer of the embodiment of the invention is applied to the wavelength division multiplexer of the embodiment, and comprises the following steps:
obtaining the cavity length of the fiber laser according to the cavity shape of the fiber laser and the repetition frequency of the pulse sequence;
acquiring a unit length dispersion value of a single-mode fiber for a set wave band and a unit length total dispersion value when the diameter of the coupling part 200 is a set diameter;
according to lightCavity length L of fiber laser and unit length dispersion value GVD of single-mode fiber for set wave band 1 And a total dispersion value GVD per unit length when the diameter of the coupling portion 200 is a set diameter 2 The length l of the coupling portion 200 is obtained.
According to the design method of the wavelength division multiplexer, a formula of the repetition frequency of a pulse sequence is determined according to the cavity shape of the fiber laser, a length formula of the cavity is obtained through conversion of the repetition frequency formula, a unit length dispersion value of a single-mode fiber for a set wave band and a unit length total dispersion value when the diameter of the coupling part 200 is the set diameter are obtained through simulation calculation, the length value of the cavity is obtained by combining the determined repetition frequency selection value, and therefore the length of the coupling part 200 is obtained, and based on an optical fiber tapering process, the first optical fiber 400 and the second optical fiber 500 form a first coupling section 410 and a second coupling section 510 meeting the diameter and length requirements of the coupling part 200 through the tapering process.
As shown in fig. 3, a method for manufacturing a wavelength division multiplexer according to an embodiment of the present invention is used for manufacturing the wavelength division multiplexer according to the above embodiment, and includes:
the tapered portions of the first optical fiber 400 and the second optical fiber 500 are disposed opposite to and parallel to each other and spaced apart from each other by a set distance d;
heating and stretching the tapered portions of the first optical fiber 400 and the second optical fiber 500 until the diameter of the tapered portions and the total taper length reach a set coupling portion length l, forming a coupling portion 200;
the coupling part 200 is mechanically packaged.
The manufacturing method of the wavelength division multiplexer is based on the optical fiber fusion tapering principle, and an optical fiber tapering machine or equipment with similar functions is needed. Selecting 2 applicable optical fibers, namely a first optical fiber 400 and a second optical fiber 500, designing a set distance between tapered parts of the first optical fiber 400 and the second optical fiber 500 used in the tapering process, forming the diameter of a coupling part 200 and the length of the coupling part 200 according to dispersion parameters of set wavelengths in the optical fibers, dispersion amounts to be compensated and the like, stripping coating layers of the tapered parts of the first optical fiber 400 and the second optical fiber 500, cleaning the surfaces of the coating layers, clamping the first optical fiber 400 and the second optical fiber 500, enabling the to-be-pushed and pulled parts to be close to the set distance, heating the tapered parts by means of oxyhydrogen flame and the like, moving a stepping motor, enabling the optical fibers to be accurately stretched, melting and tapering the tapered parts, and gradually enabling the diameters to reach the set diameters, thereby forming a part of the coupling part 200; the heating operation is repeated along the portion to be tapered (both sides) to make the length of the coupling portion 200 reach the set length, that is, the coupling portion 200 conforming to the set target is formed, and then the suspended end of the wavelength division multiplexer is cut off and packaged by using a proper method.
The first optical fiber 400 and the second optical fiber 500 may be passive single-mode optical fibers, passive double-clad optical fibers, single-clad neodymium-doped optical fibers, double-clad neodymium-doped optical fibers, etc., and the optical fibers may be polarization maintaining (polarization maintaining) or non-polarization maintaining (non-polarization maintaining) optical fibers, and the processing equipment used is an optical fiber tapering machine, a matched optical fiber fixture, an optical fiber device packaging device, a consumable, etc.
As shown in fig. 2, the optical fiber laser provided in the embodiment of the present invention includes a saturable absorber 610, a first wavelength division multiplexer 620, a single cladding neodymium-doped optical fiber 630, a second wavelength division multiplexer 640, and a laser output end 650, which are sequentially connected, where the first wavelength division multiplexer 620 and the second wavelength division multiplexer 640 are all the wavelength division multiplexers as above, and the set wavelength bands of the first wavelength division multiplexer 620 and the second wavelength division multiplexer 640 are different.
According to the fiber laser provided by the embodiment of the invention, the first wavelength division multiplexer 620 is connected into the 920 nm-band fiber laser based on the neodymium-doped fiber, so that the positive group velocity dispersion in the fiber laser can be counteracted, meanwhile, 1060-1090nm noise light is filtered, and the two functions together help to start and stabilize the mode locking of the 920nm band. The ultra-short laser pulse with the wave band of 920nm generated by the fiber laser has better application in two-photon fluorescence microscopy imaging, and can be used for high-resolution microscopy imaging in biomedical fields such as neuroscience and the like.
In this embodiment, 808nm band continuous laser enters the laser cavity through the second wavelength division multiplexer 640, and excites neodymium ions (Nd 3+ ) Obtaining 920nm wave band and 1060-1090nmThe spontaneous radiation light is mixed and enters the first wavelength division multiplexer 620 from the common terminal. After the light in the 920nm band enters the first wavelength division multiplexer 620, due to the anomalous dispersion accumulation provided by the coupling portion 200, the effect of dispersion compensation is obtained, and the light enters the next-stage element in the laser cavity along one of the output ends, that is, the saturation absorbing device 610, and the noise light in the 1060-1090nm band is filtered out of the laser cavity from the other output end. Light in the 920nm band then undergoes a saturable absorption effect at the saturable absorber 610, i.e., the higher the reflectance is the stronger the instantaneous power in the incident light; the weaker the instantaneous power, the higher the absorption. Light in the 920nm band reflected by the saturable absorber 610 returns to and passes through the first wavelength division multiplexer 620, the single-clad neodymium-doped fiber 630, and the second wavelength division multiplexer 640 to the laser output 650. The laser output end 650 outputs light with the wavelength band of 920nm according to a certain proportion, and the rest proportion is reflected back into the laser cavity by the original path to continue the next oscillation process. In this way, the fiber laser can realize laser oscillation in 920nm band, and can realize mode locking and generate ultrashort pulse under the combined action of the saturable absorption effect of the saturable absorber 610 and the dispersion compensation effect of the first wavelength division multiplexer 620.
In this embodiment, the saturable absorber 610 may be a semiconductor saturable absorber mirror or other saturable absorber based on graphene, carbon nanotube, black phosphor, etc., and the laser output 650 may be formed by a fiber optic output mirror or fiber bragg grating, and the output/reflection ratio depends on the design parameters of the laser output 650 itself.
According to one embodiment of the present invention, the first wavelength division multiplexer 620 is a 920/1060-1090nm wavelength division multiplexer and the second wavelength division multiplexer 640 is a 808/920nm wavelength division multiplexer. In this embodiment, 808nm continuous laser enters the laser cavity through 808/920nm wavelength division multiplexer, and excites neodymium ion (Nd 3+ ) The mixed spontaneous radiation light of 920nm wave band and 1060-1090nm is obtained, and enters the invented 920/1060-1090nm wavelength division multiplexer from public end. Wherein, after the light with the wave band of 920nm enters the 920/1060-1090nm wavelength division multiplexer, the light is transmitted by the coupling part 200The provided anomalous dispersion accumulation will harvest the effect of dispersion compensation and enter the next stage element in the laser cavity along one of the outputs, while noise light in the 1060-1090nm band will be filtered out of the laser cavity from the other output.
According to one embodiment of the present invention, the second wavelength division multiplexer 640 is also connected to a single-mode optical pump source 660. In this embodiment, the single-mode optical pump source 660 is a single-mode 808nm optical pump source, and 808nm continuous laser is emitted from the optical pump source and enters the 808/920nm wavelength division multiplexer.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention, and are not limiting; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit and scope of the technical solutions of the embodiments of the present invention.
Claims (5)
1. A fiber laser, characterized by: the device comprises a saturable absorption device, a first wavelength division multiplexer, a single-cladding neodymium-doped optical fiber, a second wavelength division multiplexer and a laser output end which are connected in sequence, wherein the second wavelength division multiplexer is also connected with a single-mode optical pumping source; and the first wavelength division multiplexer is different from the second wavelength division multiplexer in set wave band; the first wavelength division multiplexer is a 920/1060-1090nm wavelength division multiplexer, and the second wavelength division multiplexer is a 808/920nm wavelength division multiplexer;
the first wavelength division multiplexer and the second wavelength division multiplexer each comprise an input part, a coupling part and an output part which are sequentially connected, and the length of the coupling part is as followsWherein L is the cavity length of the fiber laser, GVD 1 GVD (Global wavelength division multiplexing) for unit length dispersion value of single-mode fiber for set wave band 2 Single for setting the diameter of the coupling part to the set diameterBit length total dispersion value;
the first wavelength division multiplexer and the second wavelength division multiplexer all comprise a first optical fiber and a second optical fiber, the first optical fiber comprises a first coupling section, an input section and a first output section, the input section and the first output section are respectively connected with two ends of the first coupling section, the second optical fiber comprises a second coupling section and a second output section connected with one end of the second coupling section, the first coupling section and the second coupling section are opposite and are mutually parallel to form the coupling part, the input section forms the input part, and the first output section and the second output section form the output part.
2. The fiber laser of claim 1, wherein: the cavity of the fiber laser is an annular cavityWhere c is the speed of light in vacuum, n is the refractive index of light propagating in the fiber, f R Is the repetition frequency of the pulse train.
3. The fiber laser of claim 1, wherein: the cavity of the fiber laser is a linear cavity, thenc is the speed of light in vacuum, n is the refractive index of light propagating in the fiber, f R Is the repetition frequency of the pulse train.
4. A method for designing a wavelength division multiplexer, which is applied to at least one of the first wavelength division multiplexer and the second wavelength division multiplexer as claimed in any one of claims 1 to 3, and comprises:
obtaining the cavity length of the fiber laser according to the cavity shape of the fiber laser and the repetition frequency of the pulse sequence;
acquiring a unit length dispersion value of a single-mode fiber for a set wave band and a unit length total dispersion value when the diameter of a coupling part is the set diameter;
according to the cavity length L of the fiber laser and the dispersion value GVD of a single-mode fiber in a unit length of a set wave band 1 And a total dispersion value GVD per unit length when the diameter of the coupling part is a set diameter 2 The length l of the coupling part is obtained.
5. A method for manufacturing a wavelength division multiplexer is characterized in that: a method for making at least one of the first wavelength division multiplexer and the second wavelength division multiplexer of any one of claims 1 to 3, comprising:
the tapered part of the first optical fiber and the tapered part of the second optical fiber are opposite and are arranged in parallel, and are separated by a set distance d;
heating and stretching the tapered part of the first optical fiber and the tapered part of the second optical fiber until the diameter of the tapered part reaches a set diameter and the length of the total tapered region reaches a set coupling part length l to form a coupling part;
the coupling portion is mechanically encapsulated.
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