CN114153018A - Reflectivity control method and manufacturing device of weak reflection grating based on movable lens system - Google Patents
Reflectivity control method and manufacturing device of weak reflection grating based on movable lens system Download PDFInfo
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- CN114153018A CN114153018A CN202111226529.1A CN202111226529A CN114153018A CN 114153018 A CN114153018 A CN 114153018A CN 202111226529 A CN202111226529 A CN 202111226529A CN 114153018 A CN114153018 A CN 114153018A
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- 238000002310 reflectometry Methods 0.000 title claims abstract description 30
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- 239000013307 optical fiber Substances 0.000 claims abstract description 36
- 238000006073 displacement reaction Methods 0.000 claims description 3
- 230000003287 optical effect Effects 0.000 abstract description 9
- 208000033999 Device damage Diseases 0.000 abstract 1
- 239000000835 fiber Substances 0.000 description 7
- 230000008859 change Effects 0.000 description 3
- 230000001427 coherent effect Effects 0.000 description 3
- 230000008878 coupling Effects 0.000 description 2
- 238000010168 coupling process Methods 0.000 description 2
- 238000005859 coupling reaction Methods 0.000 description 2
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- 238000009795 derivation Methods 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
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- 238000005259 measurement Methods 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 1
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- 238000012986 modification Methods 0.000 description 1
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- PVADDRMAFCOOPC-UHFFFAOYSA-N oxogermanium Chemical compound [Ge]=O PVADDRMAFCOOPC-UHFFFAOYSA-N 0.000 description 1
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Abstract
The invention provides a reflectivity control method and a manufacturing device of a weak reflection grating based on a movable lens system, wherein the manufacturing device comprises the following steps: the device comprises a laser, a concave lens, a convex lens, a cylindrical lens, a phase mask and a control device at least used for adjusting the position of the concave lens. Wherein, laser emitted by the laser is diffracted by the phase mask after being shaped by the concave lens, the convex lens and the cylindrical lens to generate coherence, and weak reflection gratings are engraved on the optical fiber; the invention can realize the adjustment of the grating length by at least adjusting the position of the concave lens, thereby adjusting the reflectivity of the grating. The reflectivity of the weak reflection grating is adjusted by adjusting the lens beam expanding system of the weak reflection grating manufacturing device, and compared with the traditional optical attenuator, the device damage caused by long-time laser irradiation is avoided, so that the reflectivity of the grating is adjusted under the condition of not reducing the stability of an optical fiber writing system.
Description
Technical Field
The invention relates to the field of grating manufacturing, in particular to a reflectivity control method of a weak reflection grating based on a movable lens system and a weak reflection grating manufacturing device.
Background
A weak reflection grating generally refers to a fiber grating with a reflectivity of less than 1%, and such a grating is generally used in a fiber grating sensing system based on the change of the wavelength or intensity of the reflected light of the grating with stress or temperature. Compared with the common fiber grating, the transmission loss of the grating is much lower than that of the normal grating, and meanwhile, the crosstalk effect of the reflection signals between the gratings is weak, so that the weak reflection grating can realize multi-grating series connection on a single optical fiber which cannot be realized by the common grating, the number of grating series multiplexing is greatly improved, the complexity of a fiber grating sensing system is reduced, and the development of the fiber grating sensing system is greatly promoted.
The basic principle of the manufacturing device of the common weak reflection grating is quite similar, and the refractive index change of the photoinduced optical fiber is utilized. Taking 248nm ultraviolet light as an example, 248nm laser can cause germanium-oxygen defects in the optical fiber to change, thereby causing color centers to be generated in the optical fiber, and further changing the refractive index of the optical fiber. The ultraviolet light emitted by the laser forms a coherent fringe near the mask plate through a +/-1 order diffraction fringe generated after passing through the phase mask plate or forms a coherent fringe through a Talbot interferometer, and the optical fiber is placed at the coherent fringe to form the weak reflection grating on the optical fiber. However, the reflectivity of the weak reflection grating needs to be adjusted to ensure that the reflectivity is as low as possible without reducing the sensing sensitivity of the fiber grating sensing system, so that the sensing length of the weak reflection fiber grating can be sufficiently extended. The reflectivity of the grating can be adjusted by adjusting the grating length and the intensity of the photo-induced refractive index variations, however, since the pulse energy of the laser is generally not continuously adjustable, various other ways are required to further fine-tune the pulse intensity of the laser.
At present, a continuous optical attenuator is used as a commonly used adjusting mode, but the optical attenuator can be damaged when bearing overhigh optical power, cannot be used in equipment with high optical power, and can be damaged when a weak reflecting grating is continuously manufactured for a long time, so that a new grating reflectivity adjusting method is needed to be researched.
Disclosure of Invention
The invention aims to provide a reflectivity control device based on a weak reflection grating of a movable lens system, aiming at the defects of the prior art. The present invention proposes to control the grating reflectivity by adjusting the lens system used for laser beam shaping.
The purpose of the invention is realized by the following technical scheme:
a reflectivity control method of weak reflection grating based on movable lens system, the weak reflection grating is diffracted by phase mask plate to generate coherence after laser emitted by laser is shaped by concave lens, convex lens and cylindrical lens, and is obtained by writing on optical fiber; the method specifically comprises the following steps:
the length of the grating at the position of the optical fiber is adjusted by adjusting at least the position of the concave lens (or simultaneously adjusting the convex lens and the cylindrical lens), so that the reflectivity of the weak reflection grating is adjusted. The larger the distance between the concave lens and the convex lens is, the smaller the light spot length at the optical fiber is, and the smaller the reflectivity of the weak reflection grating manufactured by the device is.
The method comprises the following steps: laser, concave lens, convex lens, cylindrical mirror, phase mask. The laser emitted by the laser is diffracted by the phase mask after being shaped by the concave lens, the convex lens and the cylindrical lens to generate coherence, the weak reflection grating is inscribed on the optical fiber, the length of the grating can be adjusted by adjusting the position of the concave lens, and then the reflectivity of the grating is adjusted.
Further, the position of the concave lens is adjusted according to the relation between the distance between the convex lens and the concave lens and the length of the light spot at the optical fiber by analyzing the optical path:
wherein:
the maleIn the formula L1Is the laser spot length, L2Is the length of the light spot at the optical fiber, d1Is the distance between the convex lens and the concave lens, d2Is the distance between the convex lens and the cylindrical lens, f0Is the focal length f of the concave lens1Is the focal length f of the convex lens2Is the focal length of the cylindrical mirror.
Further, the method also comprises the step of adjusting the positions of the convex lens and the cylindrical lens.
Further, the absolute value of m is larger than 40cm, the optimal distance between the cylindrical mirror and the optical fiber changes little at the moment and is approximately equal to the focal length of the cylindrical mirror, the absolute value of n is within +/-3 cm, and the light spot length L is at the moment2Distance d from convex lens1The reflectivity of the weak reflection grating can be accurately adjusted only by adjusting the position of the concave lens.
A weak reflecting grating manufacturing device based on the method is characterized by comprising the following steps: the device comprises a laser, a concave lens, a convex lens, a cylindrical lens, a phase mask and a control device at least used for adjusting the position of the concave lens; laser emitted by the laser is diffracted by the phase mask after being shaped by the concave lens, the convex lens and the cylindrical lens to generate coherence, and weak reflection gratings are inscribed on the optical fibers.
Further, the control device is an electric displacement table.
Further, the control device is also used for adjusting the positions of the convex lens and the cylindrical mirror.
Further, the phase mask is a uniform mask.
Compared with the prior art, the technical scheme of the invention has the following characteristics:
1. the innovative proposal uses a lens system to adjust the grating length and thus the reflectivity of the grating.
2. Because the light beam is not shielded, the device is not easy to be damaged by high-power ultraviolet light and damaged by long-time illumination, meanwhile, the grating length can be adjusted only by adjusting the position of the concave lens, the operation is simple, and the stability of the writing system is ensured.
3. By further combining the chirp mask and the real-time measurement of the grating strength, the technical scheme of the invention can be further expanded, and the technologies of grating bandwidth control, grating consistency control and the like can be realized.
Drawings
FIG. 1 is a schematic top view of a movable lens system based weak reflection grating fabrication apparatus;
FIG. 2 is a schematic side view of a moveable lens system based weak reflection grating fabrication apparatus;
FIG. 3 is a diagram showing the relationship between the distance between a convex lens and a concave lens in a weak reflection grating manufacturing device based on a movable lens system and the influence on the position of an optical fiber and the length of a light spot;
in the figure: the laser device comprises a laser (1), a concave lens (2), a convex lens (3), a cylindrical lens (4), a phase mask (5), an optical fiber (6), and a distance d between the convex lens and the concave lens1(7) Distance d between the convex lens and the cylindrical mirror2(8) Optimal distance d between cylindrical mirror and optical fiber3(9) Spot length L2(10)。
Detailed Description
Reference will now be made in detail to embodiments of the present invention (hereinafter referred to simply as embodiments), examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention. The invention is further described with reference to the following figures and specific embodiments.
rmax＝tanh2(κL) (1)
The reflectivity of the bragg reflection grating, which can be obtained from the basic knowledge of the grating, is given by formula (1), wherein κ represents the coupling coefficient of the grating and is related to the writing depth of the grating, the contrast of the grating, and L is the length of the grating. As can be seen from the equation, when κ L is sufficiently small and κ is constant, the reflectivity of the grating is approximately proportional to the square of the grating length. In theory, therefore, control of the reflectivity of the grating can be achieved by controlling the length of the grating.
Based on the above analysis, the structure of one embodiment of the present invention is shown in fig. 1. The laser device comprises a laser (1), a concave lens (2), a convex lens (3), a cylindrical lens (4) and a phase mask (5). The laser (1) is a 248nm KrF excimer laser and can emit pulse laser for grating writing, the light path under normal conditions is that the laser passes through a concave lens (2) and a convex lens (3), then expands beam into wider parallel light, then passes through a cylindrical lens (4), converges wide light spots into linear light spots, and then writes an optical fiber (6). However, in the present embodiment, the distance d between the convex lens (3) and the concave lens (2) can be obtained by a basic derivation of ray optics1(7) And the length L of the light spot2(10) By which the distance d between the convex lens (3) and the concave lens (2) can be determined by controlling1(7) Controlling the spot length L2(10) Thereby controlling the grating length and realizing the control of the grating reflectivity. It is noted that the distance d between the convex lens (3) and the concave lens (2) is varied1(7) The optimal distance d between the cylindrical mirror (4) and the optical fiber (6)3(9) I.e. the distance between the cylindrical mirror (4) and the optical fiber (6) is also changing when the optical fiber is exactly at the most concentrated position of the light beam, and needs to be considered simultaneously when designing.
In order to quantitatively describe the relationship between the above variables, the optical path needs to be analyzed, and according to the optical path diagram, the following formula can be obtained from the similarity of the triangles:
wherein L is1Is the laser (1) spot length, L2Is the length of a light spot L at the optical fiber (6)3Is the light spot length L at the right focus of the convex lens (3)4Is the light spot length L at the convex lens (3)5Is the light spot length L at the right focus of the cylindrical mirror (4)6Is the length of the light spot at the cylindrical mirror (4), d1Is the distance between the convex lens (3) and the concave lens (2), d2Is the distance between the convex lens (3) and the cylindrical mirror (4), d3Is a cylindrical mirror (4)Optimum distance from the optical fiber (6), f0Is the focal length f of the concave lens (2)1Is the focal length f of the convex lens (3)2Is the focal length of the cylindrical mirror (4). As shown in fig. 3, let m ═ f1-d2-f2，n＝f0+d1-f1The optimal distance d between the cylindrical mirror (4) and the optical fiber (6) can be obtained3(9) The relationship to n is as follows:
to d3The derivation is carried out to obtain:
due to n and the distance d between the convex lens (3) and the concave lens (2)1(7) In a linear relationship, the above formula may also be reflected by d1And d3The relationship (2) of (c). As can be seen from the formula (3), the distance d between the convex lens (3) and the concave lens (2)1(7) While moving, the optimal distance d between the cylindrical mirror (4) and the optical fiber (6)3(9) As well as are changing. Similarly, the distance d between the convex lens (3) and the concave lens (2) can be obtained1(7) And the length L of the light spot2(10) The relationship of (1) is:
wherein:
further finishing to obtain:
from the formula (4), it can be obtainedWhen the absolute value of m is large, d3Is smaller than the reciprocal of d3The variation distance of (2) is shorter, in the embodiment, the absolute value of m is more than 40 cm; from the equation (7), when n approaches 0, the spot length L is obtained2(10) A distance d between the convex lens (3) and the concave lens (2)1(7) Approximately linear, n is within + -3 cm in this embodiment.
In order to adapt to the length of the phase mask (5), the focal length of the concave lens (2) selected in this embodiment is 7cm, the focal length of the convex lens (3) is selected to be 21cm, the focal length of the cylindrical lens (4) is selected to be 5cm, and the distance d between the convex lens (3) and the cylindrical lens (4) is selected to be 7cm2(8) The value is 60 cm. Under the condition, the optimal distance d between the cylindrical mirror (4) and the optical fiber (6)3(9) And the length L of the light spot2(10) Following the distance d between the convex lens (3) and the concave lens (2)1(7) The relationship is shown in fig. 3, it being seen that, on the one hand, the distance d between the convex lens (3) and the concave lens (2)1(7) When the length of the light spot is adjusted from 12cm to 19cm, the length of the light spot is reduced from 1.1cm to about 0.5cm, meanwhile, the optimal distance between the cylindrical mirror (4) and the optical fiber (6) is only changed by 0.7cm, and from experience in practical operation, the optical path error of 0.7cm does not obviously influence the writing effect of the grating. Therefore, in the present embodiment, the entire lens system can achieve control of the grating length only by moving the concave lens (2). The control range is 0.6 cm. Since the length of the phase mask (5) used in this embodiment is 1cm, which can be obtained from the formula (1), under the condition of the weak reflection grating, the coupling coefficient κ can be approximately regarded as a constant, and it can be ensured that κ L is small, so that the grating reflectivity in this embodiment can be reduced to about 1/4 of the original grating intensity, that is, the embodiment can realize the grating reflectivity adjustment of 25% -100% of the original grating reflectivity.
In the embodiment, the position of the concave lens (2) is controlled by an electric precision displacement table, and the bandwidth control, the grating consistency control and the like of the grating can be realized by combining the chirp mask plate and the real-time measurement of the grating strength.
The above description is only a preferred embodiment of the present invention and should not be taken as limiting the invention, and any minor modifications, equivalents and improvements made to the above embodiment according to the technical spirit of the present invention should be included in the protection scope of the present invention.
Claims (8)
1. A reflectivity control method of weak reflection grating based on movable lens system, the weak reflection grating is diffracted by phase mask plate to generate coherence after laser emitted by laser is shaped by concave lens, convex lens and cylindrical lens, and is obtained by writing on optical fiber; the method is characterized by comprising the following steps:
the length of the grating at the position of the optical fiber is adjusted by adjusting at least the position of the concave lens, so that the reflectivity of the weak reflection grating is adjusted. The larger the distance between the concave lens and the convex lens is, the smaller the light spot length at the optical fiber is, and the smaller the reflectivity of the weak reflection grating manufactured by the device is.
2. The method of claim 1, wherein the position of the concave lens is adjusted according to the distance between the convex lens and the concave lens and the length of the light spot at the optical fiber:
wherein:
l in the formula1Is the laser spot length, L2Is the length of the light spot at the optical fiber, d1Is the distance between the convex lens and the concave lens, d2Is the distance between the convex lens and the cylindrical lens, f0Is the focal length f of the concave lens1Is the focal length f of the convex lens2Is the focal length of the cylindrical mirror.
3. The method of claim 2, further comprising adjusting the position of the convex lens, cylindrical lens.
4. The method of claim 2, wherein m is greater than 40cm in absolute value and n is within ± 3cm in absolute value.
5. A device for manufacturing a weak reflecting grating based on the method of any one of claims 1 to 4, comprising: the device comprises a laser, a concave lens, a convex lens, a cylindrical lens, a phase mask and a control device at least used for adjusting the position of the concave lens; laser emitted by the laser is diffracted by the phase mask after being shaped by the concave lens, the convex lens and the cylindrical lens to generate coherence, and weak reflection gratings are inscribed on the optical fibers.
6. The apparatus of claim 5, wherein the control device is a motorized displacement stage.
7. The device for manufacturing a weak reflecting grating as claimed in claim 5, wherein the control device is further used for adjusting the position of the convex lens and the cylindrical lens.
8. The apparatus of claim 5, wherein the phase mask is a uniform mask.
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Cited By (1)
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| CN115236797A (en) * | 2022-08-12 | 2022-10-25 | 武汉理工大学 | High-temperature-resistant weak-fiber grating array and preparation method thereof |
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