RU2711001C1 - Method of forming tubular channel waveguide and apparatus for its implementation - Google Patents

Abstract

FIELD: instrument engineering.SUBSTANCE: invention relates to optical instrument-making and a method of forming in a sample an optical material of a shell of a tubular channel waveguide. Method is carried out by local reduction of material refraction index in working area of neck of focused radiation of femtosecond laser, moved relative to the sample along a cylindrical spiral created by elliptical movements of the sample in the cross-section of the waveguide with axes Y-Z and longitudinal movement of the sample along the axis X of the waveguide. Laser beam axis lies in the Y-Z plane parallel to the transverse axis Z of the waveguide. Cross-section of the neck in the plane Y-Z is formed by asymmetric defocusing of the laser beam in the direction of the narrow axis Y of the neck to the size required to form an thickness of thickness of the shell along its entire perimeter.EFFECT: providing the possibility of creating a tubular channel waveguide with a uniform shell of unlimited length with minimum losses of energy transmitted through the waveguide.3 cl, 5 dwg

Description

The invention relates to optical waveguides, in particular, to tubular channel waveguides made as part of an optical material.

A known technique for the formation of optical structures in glasses and crystals by exposure to focused radiation of femtosecond lasers, including the formation of channel waveguides [1-4].

The closest in technical essence to the claimed solution is a method of forming a tubular channel waveguide in a sample of optical material by locally decreasing the refractive index of the material in the working region of the waist of the focused radiation of a femtosecond laser moving relative to the sample in a cylindrical spiral created by elliptical relative displacements of the sample in the cross section of the waveguide with the YZ axes and the longitudinal movement of the sample along the X axis of the waveguide, moreover, the axis l of the azero beam lies in the YZ plane parallel to the transverse axis Z of the waveguide, the laser pulse energy density in the working region of the laser beam waist exceeds the threshold value at which the refractive index of the material changes, and the waist scan speed is such that the regions of change in the refractive index created by successive laser pulses overlap [4].

This method is characterized by the asymmetry of the working region of the waist of the laser beam, leading to unevenness of the formed shell of the waveguide. To prevent this, laser radiation is introduced into the sample parallel to the axis of the waveguide [4, Fig 1 c)]. However, with such a solution, the waveguide length is substantially limited. In the indicated source, the length of the formed waveguide is only 4.7 mm.

An object of the present invention is to provide a tubular channel waveguide with an equal wall sheath of unlimited length with minimal losses of energy transmitted through the waveguide.

This problem is solved due to the fact that in the known method of forming a tubular channel waveguide cladding in a sample of optical material by locally reducing the refractive index of the material in the working region of the waist of the focused radiation of a femtosecond laser moving relative to the sample in a cylindrical spiral created by elliptical movements

the sample in the cross section of the waveguide with the YZ axes and the longitudinal movement of the sample along the X axis of the waveguide, moreover, the axis of the laser beam lies in the YZ plane parallel to the transverse axis Z of the waveguide, the laser pulse energy density in the working region of the laser beam waist exceeds the threshold value at which a change occurs the refractive index of the material, the waist cross section in the YZ plane is formed by asymmetric defocusing of the laser beam in the direction of the narrow waist axis Y to the size necessary to form I have the desired thickness of the shell around its perimeter.

где d - расстояние между главными плоскостями объектива и цилиндрической линзы, D_о - световой диаметр объектива, k=d_Y/d_Z, d_Y - диаметр рабочей области перетяжки в направлении оси Y, d_Z - диаметр рабочей области перетяжки в направлении оси Z.The proposed method can be implemented on the installation for forming a tubular channel waveguide in an optical material sample containing a femtosecond laser radiation source, a lens with a focal length F, a table for attaching an optical material sample with the possibility of longitudinal movement along the X axis and elliptical scanning of the sample in the ZY plane, a cylindrical lens with a focal length is introduced in front of the lens

where d is the distance between the main planes of the lens and the cylindrical lens, D _о is the light diameter of the lens, k = d _Y / d _Z , d _Y is the diameter of the working area of the waist in the direction of the Y axis, d _Z is the diameter of the working area of the waist in the Z axis direction .

отстоящая от входного зрачка объектива не более, чем на

, где λ - длина волны излучения лазера.Instead of a cylindrical lens, a slit diaphragm with a slit parallel to the X axis with a slit width can be inserted between the laser radiation source and the lens

distant from the entrance pupil of the lens no more than

where λ is the wavelength of the laser radiation.

In FIG. 1 shows the principle of the formation of the working region of the laser beam using a cylindrical lens in the longitudinal section Z-X (Fig. 1A) and in the cross section Z-Y (Fig. 1b). FIG. 2 illustrates cross-sections of the trajectory of the working area in the Z-Y plane in the absence of a cylindrical lens (Fig. 2a) and with a cylindrical lens (Fig. 2b). In FIG. 3 shows microphotographs of the formed end of the waveguide (Fig. 3a) recorded with a cylindrical lens and a side view of the waveguide (Fig. 3b). In FIG. 4 is a diagram of an apparatus for implementing the method by asymmetric defocusing of a laser beam using a cylindrical lens. FIG. Figure 5 illustrates the dependence of the width of the working region of the waist on the size of the slit diaphragm bounding the input beam along the X axis (Fig. 5a) and along the Y axis (Fig. 5b).

According to FIG. 1 or FIG. 5, the laser beam is concentrated in the working area with dimensions d _X , d _Y , d _Z , in which the energy density of the laser pulse in the working area

the waist of the laser beam exceeds the threshold value at which the refractive index of the material changes. When scanning a sample in the YZ plane, the working region of the beam forms an annular zone in the cross section of the waveguide — non-uniform (Fig. 2a) for different values of d _Y and d _Z and even for their close values (Fig. 2b). With simultaneous longitudinal movement along the X axis of the sample of optical material, a spiral trajectory of the laser beam working area is formed inside it, along which a tubular shell of the waveguide with a thickness determined by the dimensions of the working area is formed (Fig. 3a and Fig. 3b).

The installation for forming a waveguide (Fig. 4) contains a laser 1, and an optical system 2, including a cylindrical lens 3 and a lens 4, in the focus of which the working area of the waist of the laser beam 5 is formed, located in the thickness of the sample of optical material 6. The working area can make circular movement relative to the sample along trajectory 7 in a plane perpendicular to the X axis by the corresponding movements of the table 8, on which the sample is mounted, having the possibility of transverse and longitudinal movements of the sample in three coordinates there, using a three-coordinate translation table 9, controlled by a software device 10.

For asymmetric defocusing of the laser beam between the laser and the lens 4 can be installed slotted aperture 11 (Fig. 5).

Optical material 6, for example, neodymium-activated yttrium-aluminum garnet, is mounted on stage 8 so that the longitudinal feed direction coincides with the intended axis X of the optical waveguide. The radiation of laser 1 is focused through the polished surface of the sample parallel to the Z axis using optical system 2. In this case, the working area 5, formed by the constriction of the focused laser beam, is placed relative to the sample so that when it is elliptically provided by the translation table 9, the trajectory of the working area coincides with the waveguide sheath zone. Each time a laser emits a femtosecond pulse, a section of optical material is formed in the working region of the laser beam, characterized by a lower value of the refractive index. If the scanning speed of the waist is sufficiently small, so that the areas of change in the refractive index created by individual laser pulses overlap, the simultaneous longitudinal supply of sample 6 allows the formation of a tubular shell in the form of a cylindrical spiral (Fig. 3).

The thickness of the shell depends on the shape of the waist 5 of the laser beam. Usually the working area of the waist is elongated along the Z axis of the laser beam, and with its circular

the motion of the shell thickness is different in the direction of the axes Y and X (Fig. 2A). In the direction of the Y axis, the cladding may turn out to be too thin, which will lead to losses in energy transfer along the waveguide.

According to the proposed solution, the constriction of the laser beam 1 is expanded along the Y axis by its asymmetric defocusing, which ensures the required uniform thickness of the shell in its cross section (Fig. 2b). Defocusing is achieved using a cylindrical lens 3 (Fig. 4), the generatrix of which is parallel to the X axis, or using a slit diaphragm 11, oriented parallel to the X axis (Fig. 5).

A cylindrical lens introduces astigmatism into the laser beam, so that two constrictions form after the lens, the first at the focal length of the lens F, as in the absence of a cylindrical lens, and the second at a distance F _z . In FIG. 1 shows an example for the case of a negative cylindrical lens. In this case, the first hauling is working. The distance to the second constriction F _z can be calculated in the approximation of geometric optics from the relation: [5]

where F _c is the focal length of the cylindrical lens (the signs in the formula (1) take into account the negative optical power of the cylindrical lens);

d is the distance between the main planes of the lens and the cylindrical lens in the Z-Y plane.

From (1) it follows

The distance F _{Z is} determined by the necessary degree of defocusing of the laser beam to create a working area with a given ratio of dimensions d _Z and d _Y (Fig. 1).

According to the constructions of FIG. 1b)

where D _o - the light diameter of the lens, whence

Given the given design ratio d _Y = kd _Z , the calculated formula

Next, for the focal length (2) of the cylindrical lens, we find:

Example 1

F = 3.6 mm (determined by the parameters of the laser beam and optical material); D ₀ = 4.7 mm; d = 50 mm; d _z = 0.01 mm; 0.5≤k≤2.

With these data, according to (5), 3.604 mm ≤ F _Z ≤ 3.615 mm.

And in accordance with (6)

173 mm ≤ F ₄ ≤ 1642 mm.

Restriction beam Y axis width slit diaphragm also enables the construction of an elliptic cross section constriction of the beam after the focusing lens, the diameter of the constriction along the axis X is the same as that in the absence of the slit diaphragm and the diameter of the constriction of Y d _Y axis calculated by the formula for the diameter constriction Gaussian the beam in the focus of the lens, taking into account the decrease in the effective beam size in front of the lens along the Y axis due to the action of the gap [6]:

where λ is the laser radiation wavelength, h _Y is the width of the slit aperture, k is the design ratio for the ellipticity of the waist, F is the focal length of the lens.

The slit aperture must be installed at a lens distance d _h not exceeding half the Rayleigh length so that the diffraction divergence of the beam increases the size of the beam limited by it at the entrance to the lens is not significant.

Example 2

что более чем d_h=30 мм.F = 3.6 mm; d _Z = 0.01 mm, d _h = 30 mm, λ = 1030 nm. Provided that 0.5≤k≤2, the width of the working area along the Y axis should be in the range of 0.005 <d _y <0.02. Further, in accordance with (7), for the slit width, we obtain 0.23 mm <h _Y <0.92 mm, with half the Rayleigh length

which is more than d _h = 30 mm.

Thanks to this technical solution, the waveguide sheath is formed by a more uniform tool of the working area of the laser beam waist, the ellipticity of which is determined by the design ratio k, which ensures a high transmission coefficient of the waveguide at any length. In this case, the dimensions

work areas are minimally necessary and sufficient to create the required optical structure.

The measured transmittance of the waveguide does not exceed 1 dB / cm, which corresponds to the best achievements in this field.

This ensures the fulfillment of the objective of the invention is the creation of a tubular channel waveguide with an equal wall sheath of unlimited length with minimal loss of energy transmitted through the waveguide.

Sources of information

1. Patent WO 2005040874. Laser inscription of optical structures in crystals. 05/06/05.

2. US Pat. 7,132,223. Laser-written cladding for waveguide formations in glass. 11/7 06.

3. US Pat. 10, 201.874. Method and apparatus for realizing tubular optical waveguides in glass by femtosecond laser direct writing. 12/02/19.

4. Gabriela Salamu, Florin Jipa, Marian Zamfirescu, and Nicolaie Pavel 1. Cladding waveguides realized in Nd: YAG ceramic by direct femtosecond-laser writing with a helical movement technique. Optical Materials Express, Vol. 4, No. 4. p. 792. - prototype.

5. Panov V.A., Kruger M.Ya., Kulagin V.V. et al. Handbook of the designer of optical-mechanical devices. Under the total. ed. V.A. Panova. - 3rd ed., Revised. and add. - L .: Mechanical Engineering, 1980 .-- 742 p.

6. K. Moh, Y. Tan, H.-C. Yuan, D. Low, and Z. Li, "Influence of diffraction by a rectangular aperture on the aspect ratio of femtosecond direct-write waveguides," Optics Express Vol. 13, 7288-7297 (2005).

Claims (5)

1. A method of forming a tubular channel waveguide sheath in a sample of optical material by locally reducing the refractive index of the material in the working region of the waist of focused radiation of a femtosecond laser moving relative to the sample in a cylindrical spiral created by elliptical movements of the sample in the cross section of the waveguide with YZ axes and longitudinal movement of the sample along the X axis of the waveguide, the axis of the laser beam lying in the YZ plane parallel to the transverse axis Z of the waveguide, the laser pulse energy in the working region of the laser beam waist exceeds the threshold value at which the refractive index of the material changes, characterized in that the waist cross section in the YZ plane is formed by asymmetric defocusing of the laser beam in the direction of the narrow waist axis Y to the size necessary for the formation of an equal thickness shells around its perimeter. 2. Installation for forming a sheath of a tubular channel waveguide in an optical material sample according to claim 1, comprising a femtosecond laser radiation source, a lens with a focal length F, a table for attaching an optical material sample with the possibility of longitudinal movement along the X axis and elliptical scanning of the sample in the ZX plane characterized in that a cylindrical lens with a focal length is introduced in front of the lens

where d is the distance between the main planes of the lens and the cylindrical lens, D _o is the light diameter of the lens, k = d _Y / d _Z , d _Y is the diameter of the working area of the waist in the direction of the Y axis, d _Z is the diameter of the working area of the waist in the Z axis direction . 3. Installation for forming a shell of a tubular channel waveguide in an optical material sample according to claim 1, containing a femtosecond laser radiation source, a lens with a focal length F, a table for attaching an optical material sample with the possibility of longitudinal movement along the X axis and elliptical scanning of the sample in the ZX plane , characterized in that between the laser radiation source and the lens at a distance from the entrance pupil of the lens is not more

a slit diaphragm is introduced with a slit parallel to the X axis, with a slit width h _Y =

where X is the laser radiation wavelength, k = d _Y / d _Z , d _Y is the diameter of the working area of the waist in the direction of the Y axis, d _Z is the diameter of the working area of the waist in the direction of the Z axis.

Images