An ultra-low birefringence glass waveguide prepared by femtosecond laser direct writing technology, method and its application

Abstract

The present invention discloses an ultra-low birefringence glass waveguide prepared by femtosecond laser direct writing technology, a method thereof, and applications thereof, belonging to the technical field of laser processing. The present invention adopts a double compensation scheme of shape-stress based on femtosecond laser processing technology; first, the shape compensation scheme is to use a cylindrical lens-slit shaping technology to focus the femtosecond laser beam into a circular spot on the glass material, thereby preparing a waveguide with a circular end face inside Corning glass; secondly, the compensation scheme for asymmetric stress birefringence specifically refers to additionally processing a modification line at the horizontal radial neighborhood position of the circular waveguide to compensate for the asymmetric stress generated inside when processing the main waveguide with a circular end face; during the stress compensation process, the added modification line hardly affects the basic characteristics such as the transmission loss and single-mode transmission of the circular waveguide; finally, the modification line and the main waveguide with a circular end face are combined to form a new waveguide, and the new waveguide has ultra-low birefringence (1*10 ‑9 ).

Description

Technical Field

[0001] The present invention belongs to the technical field of laser processing, and particularly relates to a method for preparing an ultra-low birefringence waveguide by using a femtosecond laser direct writing technology to propose a shape-stress double compensation scheme, which solves the problems of polarization rotation of photon states and polarization mode dispersion existing in the process of light transmission in the waveguide. Background Art

[0002] The femtosecond laser direct writing technology has attracted attention due to its powerful three-dimensional processing ability and multi-material applicability. Especially for its powerful processing ability for hard transparent materials, this technology is often used to process materials such as sapphire, glass and diamond. Among them, it has been proved that using the femtosecond laser direct writing technology to process waveguides in a glass matrix can prepare optical waveguides with low loss and good modes, and related research has been widely applied in fields such as quantum computing, quantum chips, and quantum topology. The principle of the femtosecond laser direct writing optical waveguide is to focus a high-repetition-rate pulsed laser inside the glass material. The interval time between laser pulses is less than the relaxation time of electrons and phonons in the material. Eventually, heat accumulation is formed in the focused area, causing the glass at this position to melt, and finally cooling to form a core layer with a relatively high refractive index. In the non-focused area, since the material modification threshold is not reached, it will not be affected, thus forming a cladding with a low refractive index. Light is totally reflected and transmitted inside such a fiber-like structure to achieve the light guiding characteristic. Generally, when an unshaped femtosecond laser is focused inside Corning glass, due to the defocusing problem, the light spot inside the material is elliptical, so the optical waveguide processed by using this light spot is also elliptical and has a strong birefringence effect. At the same time, there are two main sources of birefringence in the waveguide. One is the birefringence caused by the waveguide shape, and the other is the birefringence caused by the internal stress in the waveguide. The strong birefringence of the waveguide will cause problems such as polarization state rotation and polarization mode dispersion of the light transmitted in the glass waveguide. More seriously, it will even affect the purity of photon states and the interference visibility in quantum tests.

[0003] In response to the above problems, the commonly used adjustment method is to add polarization rotators (wave plates) at key positions in the chip, but this cannot fundamentally solve the problem. The large-scale use of polarization rotators will cause device redundancy on the chip. In order to solve the waveguide birefringence caused by the waveguide shape, people usually use a shaped beam to focus internally to obtain a circular spot, and try to make a circular waveguide inside the Corning glass to reduce the birefringence caused by the waveguide shape. However, since the glass waveguide will inevitably involve nonlinear interactions between the laser and the material during the molding process, including the rapid melting and condensation of the dielectric material, which exhibits different degrees of stress birefringence, the circular waveguide prepared by beam shaping still has stress-induced waveguide birefringence. Therefore, in response to the birefringence caused by stress, people usually use thermal annealing to release the stress in the waveguide, but this method is not selective and will thermally anneal all waveguides and their devices in the chip. For example, the thermal annealing operation will cause the performance of devices such as the polarization beam splitter in the chip that rely on the strong birefringence effect of the waveguide to degrade or fail. Moreover, the above two types of solutions only control the waveguide birefringence value to 10 -5 -10 -6 This means that when the chip reaches the centimeter level, the negative impact of waveguide birefringence during the transmission of light in the waveguide cannot be ignored.

[0004] Currently, there is no report on improving waveguide birefringence by simultaneously addressing both shape and stress. In summary, it is urgent to find a femtosecond laser processing method that can simultaneously address waveguide birefringence caused by both shape and stress. Summary of the Invention

[0005] In order to solve the problems of polarization rotation of photon states and polarization mode dispersion in the transmission process of light in the waveguide in the prior art, the present invention provides an ultra-low birefringence glass waveguide and method prepared based on femtosecond laser direct writing technology. The present invention adopts a shape-stress dual compensation scheme based on femtosecond laser processing technology; first, the shape compensation scheme is to use cylindrical lens-slit shaping technology to focus the femtosecond laser beam on the glass material into a circular spot, thereby preparing a waveguide with a circular end face inside the Corning glass; second, the compensation scheme for asymmetric stress birefringence specifically refers to additionally processing a modification line at the horizontal radial neighborhood position of the circular waveguide to compensate for the asymmetric stress generated inside the circular end face main waveguide when processing the circular end face main waveguide; during the stress compensation process, the added modification line has almost no effect on the basic characteristics of the circular waveguide such as transmission loss and single-mode transmission; finally, the modification line is combined with the circular end face main waveguide to form a new waveguide, and the new waveguide has ultra-low birefringence (1*10 -9 ).

[0006] The present invention is achieved through the following technical solutions:

[0007] In a first aspect, the present invention provides an ultra-low birefringence glass waveguide prepared by femtosecond laser direct writing technology. The ultra-low birefringence glass waveguide consists of a main waveguide with a circular end face inside Corning glass and a modified line parallel thereto. The main waveguide with the circular end face is prepared by a cylindrical lens-slit beam shaping method, and the modified line is prepared by moving the focused laser focus along the direction parallel to the main axis of the main waveguide.

[0008] Further, the birefringence magnitude of the ultra-low birefringence glass waveguide is 1. . -9 00*10, the length of the waveguide is 25 - 100 mm; the diameter of the circular end face of the waveguide can be flexibly prepared according to the wavelength of the transmitted light at 6 - 15 μm.

[0009] In a second aspect, the present invention also provides a method for preparing an ultra-low birefringence glass waveguide by femtosecond laser direct writing technology, which specifically includes the following steps:

[0010] Step 1: Waveguide shape compensation:

[0011] The output laser of the femtosecond laser passes through a half-wave plate W1 and a Glan prism GL respectively, and is normally incident on a lens group. The lens group expands and parallelly emits the light spot, and then is reflected by a mirror group and reflected on a customized slit. The light beam passing through the slit is called a shaped light beam; the shaped light spot is incident on an objective lens OL, and is focused by the objective lens OL inside the Corning glass on the sample stage to form a circular light spot; the sample stage is leveled and its movement is controlled to make the laser focus move inside the Corning glass; the main waveguide with a circular end face is prepared by using the focused circular light spot, so as to realize the control of the waveguide shape and reduce the waveguide birefringence caused by the asymmetric shape; during the waveguide preparation process, a monitoring system is used to monitor the laser writing process;

[0012] Step 2: Waveguide stress compensation:

[0013] After the main waveguide with a circular end face is prepared by using the shaped light beam, keeping the original optical path and beam shaping means unchanged, the focused laser focus is moved along the direction parallel to the main axis of the main waveguide to process a modified line with an end face diameter smaller than that of the main waveguide; the spatial positions of the modified line and the main waveguide overlap in the radial direction, and their lengths are equal, so as to ensure that the stress generated when writing the modified line compensates the existing asymmetric stress in the main waveguide; finally, a new waveguide formed by the combination of the overlapping main waveguide and modified line is the prepared ultra-low birefringence waveguide.

[0014] Further, the femtosecond laser in Step 1 emits laser with a wavelength of 515 - 1030 nm, a repetition frequency of 1 MHz, and an output optical power of 0.1 - 5 W.

[0015] Further, the lens group described in step one is composed of a first cylindrical lens CL1 and a second cylindrical lens CL2, the mirror group is composed of a first mirror M1, a second mirror M2 and a third mirror M3, and the output laser of the femtosecond laser passes through a half-wave plate W1 and a Glan prism GL respectively, and is normally incident on the first cylindrical lens CL1 and the second cylindrical lens CL2. The lens group expands the light spot by 1-20 times and emits it parallelly; then it passes through the first mirror M1, the second mirror M2 and the third mirror M3 in sequence and is normally incident on a customized slit. <> <>

[0016] Further, the width of the customized slit described in step one is 0.2-4mm. <> <>

[0017] Further, in step one, the laser focus point is inside the Corning glass, and a circular end face main waveguide is processed at a position 160-230μm away from the surface of the Corning glass. The processing power range of the circular end face main waveguide is 300-460mw, and the direct writing speed is 10-40mm / s. <> <>

[0018] Further, the monitoring system described in step two is composed of an illumination module and a monitoring module. The illumination module is composed of a white light source and a second convex lens L2. The white light source is placed at the focal point of the second convex lens L2, and the second convex lens L2 is placed between the sample stage and the light source. The focal length of the second convex lens L2 is 5cm, so that the divergent light emitted by the white light source becomes parallel light through the second convex lens L2 and illuminates the entire sample; the monitoring module is composed of a fourth mirror M4, a first convex lens L1 and a camera CCD. The three are on the same straight line. The fourth mirror M4 reflects the sample information carried by the illumination light and converges it onto the photosensitive chip of the camera CCD through the first convex lens L1 to achieve imaging. <> <>

[0019] Further, the direct writing power of the modification line described in step two is 20-150mw, and the diameter of the modification line is 20-70% of the diameter of the circular end face main waveguide; the perpendicular distance between the starting point of the modification line processing and the main axis of the circular end face main waveguide varies between 0.05-0.5 times the diameter of the circular end face main waveguide, so that the best stress compensation distance can be found, and it can also prevent the distance between the modification line and the circular end face main waveguide from being too far and causing coupling. <> <>

[0020] In a third aspect, the present invention also provides an application of an ultra-low birefringence glass waveguide prepared by femtosecond laser direct writing technology in the preparation of directional couplers. <> <>

[0021] Compared with the prior art, the advantages of the present invention are as follows: <> <>

[0022] 1. A method for fabricating an ultra-low birefringence glass waveguide based on femtosecond laser direct writing technology proposed by the present invention has the advantage of fabricating an ultra-low birefringence waveguide; a shape-stress double compensation technology based on femtosecond laser direct writing is proposed for the first time. A circular-end face main waveguide is fabricated using a shaped beam, and a modification line is processed in the vicinity of the main waveguide with reduced laser power to compensate for the internal stress in the circular-end face main waveguide. Fundamentally, the problem of large birefringence in Corning glass waveguides is solved; at the same time, the modification line hardly affects the basic characteristics such as the transmission loss and single-mode transmission mode of the circular-end face main waveguide.

[0023] 2. The present invention has the advantage of the ability to depolarize birefringence at specific positions; since the modification line does not affect other waveguides and devices around it, and at the same time, customized processing can be realized for waveguide devices with low birefringence requirements at specific positions within the chip. Compared with the thermal annealing technology, polarization-sensitive functional devices on the chip can be flexibly retained, and it has the selectivity of depolarizing waveguide birefringence. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. In all the drawings, similar elements or parts are generally denoted by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0025] Figure 1 It is a schematic diagram of a processing device for fabricating a Corning glass waveguide based on femtosecond laser direct writing technology of the present invention;

[0026] a is a schematic diagram of the optical path; b is a cross-sectional view of the light spot after passing through the Glan prism; c is a cross-sectional view of the light spot after passing through the first mirror; d is a cross-sectional view of the light spot after passing through the third mirror;

[0027] Among them, W1 - half-wave plate, GL - Glan prism, CL1 - first cylindrical lens, CL2 - second cylindrical lens, L1 - first convex lens, L2 - second convex lens, M1 - first mirror, M2 - second mirror, M3 - third mirror, M4 - fourth mirror, Slit - slit, OL - objective lens, R - illumination light source, CCD - camera;

[0028] Figure 2 It is a schematic diagram of the processing steps and structure of the shape-stress double compensation scheme of the present invention;

[0029] Among them, (a): The circular end-face main waveguide is fabricated using a cylindrical lens-slit beam shaping technique; a decorative line is added in the vicinity of the circular end-face main waveguide to achieve stress compensation for the circular end-face main waveguide, and the combination of the two forms a waveguide with ultra-low birefringence; the length of all waveguides is 25 mm; (b): Microscope pictures of the circular end-face main waveguide and its mode field size; (c): Microscope pictures of the circular end-face main waveguide + decorative line and the mode field size of the new waveguide formed by it;

[0030] Figure 3 This is a data graph showing the variation of the birefringence of the waveguide prepared by shape-stress double compensation in the present invention with the radial distance between the decorative line and the circular end-face main waveguide;

[0031] In the figure: (a) shows the measurement method of waveguide birefringence; (b)-(e) show the ratio of the power of the component perpendicular to the input polarization light to the total power after the linearly polarized light passes through waveguides of different lengths when the decorative line and the circular end-face main waveguide change horizontally (Δd1 = 0.5 μm (b), Δd2 = 0.75 μm (c), Δd3 = 1 μm (d), Δd4 = 1.25 μm (e)); the measured waveguide birefringence can be deduced through the functional relationship; compared with a normal circular waveguide (7.12*10 -6 ), a waveguide with a birefringence value of 1.00*10 -9 can be prepared using the shape-stress double compensation scheme;

[0032] Figure 4 This is a data graph comparing the splitting ratios of directional couplers fabricated with waveguides having relatively high birefringence and ultra-low birefringence in the present invention for their sensitivity to horizontally and vertically polarized light;

[0033] Among them: The black squares represent the measured values of the splitting ratio of the directional coupler when horizontally polarized light is input into the directional coupler; the black curve represents the relationship between the transmittance of the directional coupler and the coupling length; the light-colored circles represent the measured values of the splitting ratio of the directional coupler when vertically polarized light is input into the directional coupler; the light-colored curve represents the relationship between the transmittance of the directional coupler and the coupling length; among them, (a) is the directional coupler fabricated with the ultra-low birefringence waveguide, and (b) is the directional coupler fabricated with the waveguide having relatively high birefringence; it can be seen from figures (a) and (b) that birefringence is sensitive to the polarization of the input light, and (a) proves that the ultra-low birefringence waveguide prepared by the method of the present invention has the characteristic of polarization independence and achieves the expected purpose. Detailed implementation manners

[0034] To clearly and completely describe the technical solutions and their specific working processes of the present invention, in combination with the accompanying drawings of the specification, the detailed implementation manners of the present invention are as follows:

[0035] The following embodiments quantitatively illustrate the related experiments of the shape-stress dual compensation technology based on femtosecond laser direct writing for fabricating ultra-low birefringence waveguides.

[0036] A processing method for fabricating ultra-low birefringence waveguides based on the shape-stress dual compensation technology of femtosecond laser direct writing can fabricate ultra-low birefringence waveguides with a birefringence magnitude of 1.00*10 -9 which is beneficial to realizing polarization-independent transmission of directional couplers. Secondly, it also has the advantage of birefringence processing at specific positions, which can eliminate the polarization-dependent characteristics of waveguides at specific positions and improve the visibility of on-chip quantum polarization state interference. At the same time, this method does not increase the size and complexity of the device and is conducive to integration into 3D large-scale waveguide networks. In the future, the present invention can be applied to fields such as quantum computing and topology to solve the problems of polarization encoding and entanglement in quantum photonic integrated circuits and polarization mode dispersion in traditional optical communication systems.

[0037] Example 1

[0038] As shown in (a) of Figure 2 , this embodiment provides an ultra-low birefringence glass waveguide fabricated based on femtosecond laser direct writing technology. The ultra-low birefringence glass waveguide consists of a main waveguide with a circular end face inside Corning glass and a modification line parallel to it. The main waveguide with a circular end face is fabricated by the cylindrical lens-slit beam shaping method, and the modification line is fabricated by moving the focused laser focus along the direction parallel to the main axis of the main waveguide to achieve stress compensation of the main waveguide with a circular end face. The combination of the two forms an ultra-low birefringence waveguide;

[0039] The birefringence magnitude of the ultra-low birefringence glass waveguide is 1.00*10 -9 , the length of the waveguide is 25 mm; the diameter of the main waveguide with a circular end face is 6.2 μm; the diameter of the circular end face of the modification line is 2.8 μm; Δd1 in the figure represents the radial distance between the modification line and the main waveguide with a circular end face is 0.5 μm; and so on, Δd2 = 1 μm, Δd3 = 2 μm, and Δd4 = 3 μm.

[0040] Compared with a normal circular waveguide (7.12*10 -6 ), a waveguide with a birefringence value of 1.00*10 -9 can be fabricated by adopting the shape-stress dual compensation scheme.

[0041] Example 2

[0042] This embodiment provides a method for fabricating an ultra-low birefringence glass waveguide based on femtosecond laser direct writing technology. The specific steps are as follows:

[0043] (1) Femtosecond laser power adjustment:

[0044] The femtosecond laser power adjustment utilizes a power adjustment module; the power adjustment module includes a half-wave plate W1 and a Glan prism GL (the light spot is as shown in the A cross-section of Figure 1 ). The half-wave plate W1 is fixed on an electrically controlled rotary table, and the rotation angle of the electrically controlled rotary table is adjusted through computer software; an external power meter is connected to the entrance pupil of the objective lens OL as a calibration method for the processing energy; when the laser is not turned on, the calibrated power is 0 mw under darkroom conditions; when the laser is turned on, the half-wave plate is rotated by the electrically controlled rotary table, and the measured power at the entrance pupil of the objective lens is 420 mw (direct writing power); the rotation of the electrically controlled rotary table can achieve an error control accuracy of 0.5 mw / 100 mw. Note: Each time of processing, an external power meter needs to be used to detect the laser power at the entrance pupil of the objective lens as the processing power.

[0045] (2) Femtosecond laser beam shaping:

[0046] The beam shaping module includes a first cylindrical lens CL1, a second cylindrical lens L2, a first mirror M1, a second mirror M2, a third mirror M3 and a slit. A laser beam can obtain a parallel laser beam magnified 5 times in the X direction after passing through the cylindrical lens group (the light spot is as shown in the B cross-section of Figure 1 ), and is incident normally and passes through the slit after passing through the first mirror M1, the second mirror M2 and the third mirror M3 to form a shaped beam (the light spot is as shown in the C cross-section of Figure 1 ); specifically, the light spot emitted from the laser is a circular light spot with a diameter of 1.9 mm (as shown in b of Figure 1 ), and the light beam passing through the cylindrical lens group (composed of the first cylindrical lens CL1 and the second cylindrical lens CL2) becomes an elliptical light spot of 9.6 mm * 1.9 mm (as shown in c of Figure 1 ). After the light beam passes through the first mirror M1, the second mirror M2 and the third mirror M3, it is incident normally and passes through a slit with a width of 0.84 mm, and finally the shaped beam is a slit beam of 9.6 mm * 0.84 mm (as shown in d of Figure 1 ).

[0047] (3) Adjustment of the femtosecond laser focusing direct writing illumination module:

[0048] Place the white light source at the bottom of the sample stage, and place the white light source at the focal point of the second convex lens L2. The second convex lens L2 is placed between the sample stage and the light source. The focal length of the second convex lens L2 is 5 cm, so that the divergent light emitted from the white light source becomes parallel light after passing through the second convex lens L2 and illuminates the entire sample.

[0049] (4) Adjustment of the femtosecond laser focusing direct writing monitoring module:

[0050] After the illumination light passes through the sample, the relevant information is collected by the objective lens. After passing through the third mirror M3, it is reflected by the fourth mirror M4 and passes through the first convex lens L1 to the CCD. Through the CCD camera, the surface of the sample can be clearly seen. Therefore, the leveling and direct writing processes on the sample stage can be monitored in real time on the computer side.

[0051] (5) Sample stage leveling:

[0052] The working distance of the leveling objective lens OL used is 1.7 mm, NA = 0.7, and the magnification is 40 times. The cylindrical lens-slit shaped light spot enters the objective lens OL and is focused by the objective lens OL and then incident on the sample stage. Then, a glass sample is placed on the sample stage, and the leveling process is monitored in combination with the focused direct writing monitoring module. By adjusting the rotation angle of the sample stage in the X-axis direction until a uniform damage line with a length of 10 cm and a width of 2 μm starting from the point (5 cm, 0) and ending at the point (-5 cm, 0) can be observed on the surface of the glass slide in the X direction of the laser; then, adjust the rotation angle of the sample stage in the Y-axis direction until a uniform damage line with a length of 10 cm and a width of 2 μm starting from the point (0, 5 cm) and ending at the point (0, -5 cm) can be observed on the surface of the glass slide in the Y direction of the laser; it means that the sample stage is leveled.

[0053] (6) Preparation of ultra-low birefringence waveguide based on the shape-stress double compensation scheme:

[0054] Place the bulk material to be processed - Corning glass on the sample stage, control the movement of the sample stage, and through the previous step, find the surface of the sample and control the objective lens OL to descend 170 μm so that the laser is focused inside the bulk material. The relevant femtosecond laser direct writing parameters are that the laser power after the slit is 420 mw and the direct writing speed is 40 mm / s. The sample stage moves in the X direction to directly write the circular end face main waveguide. Subsequently, adjust the modification power to 360 mw, and at the same time make the laser focusing starting point move along the horizontal radial direction of the center of the circular end face main waveguide by Δd1 = 0.50 μm, Δd2 = 0.75 μm, Δd3 = 1.00 μm, Δd4 = 1.25 μm respectively, and also directly write at a speed of 40 mm / s, as Figure 2 shown in (a). Correspondingly, when there is only the circular end face main waveguide, the waveguide end face (5.3 * 5.2 μm) and the mode field size (6.8 * 6.6 μm) are as Figure 2 shown in (b). When a modification line is added along the horizontal radial direction of 1.25 μm to the circular end face main waveguide, the combined waveguide end face (5.3 * 5.2 μm) and the mode field size (6.8 * 6.7 μm) are as Figure 2 shown in (c).

[0055] (7) Test the birefringence value of the waveguide;

[0056] After the processing is completed, the waveguide end face of the sample is polished, and the birefringence of waveguides with different parameters is measured and compared using a birefringence measurement system. Specifically, the test system is as shown in Figure 3 (a). The birefringence measurement system includes a laser that outputs 808 nm as the light source; a polarization beam splitter and a half-wave plate are used to prepare the required polarized light; an objective lens is used to couple light into the waveguide or collect the light transmitted by the waveguide; the polarization beam splitter separates the polarization of the output light to obtain light with the desired polarization direction. The specific birefringence test principle is that the birefringence of the waveguide will cause the polarization of the input light to rotate. By measuring the relationship between the output polarized light and the input polarized light of waveguides with different lengths, the magnitude of the birefringence can be inferred. It should be noted that waveguides with ultra-low birefringence will not change the polarization state of the input light.

[0057] As shown in Figure 3 (b), when the radial distance between the modification line and the main waveguide of the circular end face is 0.5 μm, the birefringence of the combined waveguide can be obtained from the curve as 7.12×10 -6 . Correspondingly, as shown in Figure 3 (c) and (d), when the radial distances are 0.75 μm and 1 μm, the birefringences of the combined waveguide are 3.90×10 -6 and 8.34×10 -7 , respectively. It can be observed that as the modification line moves away from the main waveguide of the circular end face in the radial direction, the birefringence value of the combined waveguide continuously decreases. When the radial distance between the modification line and the main waveguide of the circular end face is 1.25 μm, an optical waveguide with a birefringence as low as 1×10 -9 can be obtained. Even if the waveguide is as long as 5 cm, it will not change the polarization direction of the input polarized light, as shown in Figure 3 (e).

[0058] Through the above process, the preparation of a waveguide with ultra-low birefringence (1×10 -9 ) is successfully realized, and its quantitative characterization is carried out; the ultra-low birefringence waveguide will be widely used in polarization coding and entangled quantum photonic integrated circuits.

[0059] Example 3

[0060] This example provides an application of an ultra-low birefringence glass waveguide prepared based on femtosecond laser direct writing technology in the preparation of polarization-independent directional couplers, which specifically includes the following steps:

[0061] (1) Femtosecond laser power adjustment; the same as in Example 2.

[0062] (2) Femtosecond laser beam shaping: the same as in Example 2.

[0063] (3) Femtosecond laser focused direct writing illumination module adjustment: the same as in Example 2.

[0064] (4) Adjustment of the femtosecond laser focusing direct writing monitoring module: The same as in Embodiment 2.

[0065] (5) Levelling of the sample stage; The same as in Embodiment 2.

[0066] (6) Femtosecond laser direct writing polarization-independent directional coupler:

[0067] Place the bulk material to be processed - Corning glass on the sample stage, control the movement of the sample stage, perform direct writing of the waveguide structure, and use the ultra-low birefringence waveguides prepared in Embodiment 2 to form a directional coupler. The direct writing power is 420 mw, the direct writing speed is 40 mm / s, the direct writing spacing using the air-bearing translation of the sample stage is 250 μm, the coupling spacing is 12 μm, and the directional couplers with a bending radius of 40 mm have coupling lengths of 0 mm, 0.5 mm, 1.5 mm, 3.0 mm, 5.0 mm, 7.0 mm, 9.0 mm, 11.0 mm, 13.0 mm, and 15.00 mm respectively.

[0068] (7) Measure the polarization dependence of the directional coupler;

[0069] After completing the direct writing of multiple groups of directional couplers, polish the waveguide end faces of the samples. Couple horizontally polarized and vertically polarized light into the directional coupler through an objective lens, and use a power meter to measure the variation of the splitting ratio of multiple groups of directional couplers with the coupling region length at the output end of the directional coupler. As Figure 4 shown in (a) of -1 , for horizontally polarized light and vertically polarized light, the coupling coefficients of the polarization-independent directional coupler are 0.165 mm -1 and 0.168 mm -1 respectively. From Figure 4 in (a), the reflectivity difference between two orthogonal polarizations can be calculated as |R V -R H |≈2%, and the corresponding coupling coefficient difference is less than 0.003 mm -1 . Compared with the directional coupler composed of waveguides with relatively high birefringence (10 -5 ), as Figure 4 shown in (b) of Figure 4 , for the variation relationship between the splitting ratio and the coupling length of the directional coupler composed of ordinary birefringent waveguides, it can be judged that for horizontally polarized light and vertically polarized light, the coupling coefficients of the polarization-independent directional coupler are 0.218 mm -1 and 0.176 mm -1 respectively, and the corresponding coupling coefficient difference is 0.042 mm -1 , and this directional coupler is polarization-dependent. That is, the ultra-low birefringence waveguide can assist in preparing a polarization-independent directional coupler, which is an important device in the application of quantum state or polarization-entangled photon polarization encoding.

[0070] The preferred embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0071] In addition, it should be noted that, in the various specific technical features described in the above specific embodiments, they can be combined in any suitable manner without conflict. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

[0072] Furthermore, any combination can be made between various different embodiments of the present invention, as long as it does not violate the idea of the present invention, and it should also be regarded as the content disclosed by the present invention.

Claims

1. An ultra-low birefringence glass waveguide prepared by femtosecond laser direct writing technology, characterized in that, The ultra-low birefringence glass waveguide is composed of a main waveguide with a circular end face inside Corning glass and a modified line parallel to it. The main waveguide with the circular end face is prepared by the cylindrical lens-slit beam shaping method, and the modified line is prepared by moving the focused laser focus along the direction parallel to the main axis of the main waveguide; The birefringence level of the ultra-low birefringence glass waveguide is 1.00*10 -9 , the length of the waveguide is 25-100 mm; the diameter of the circular end face of the waveguide can be flexibly prepared according to the wavelength of the transmitted light at 6-15 μm; Among them, the ultra-low birefringence glass waveguide prepared based on the femtosecond laser direct writing technology is prepared by the following method, and the method specifically includes the following steps: Step 1: Waveguide shape compensation: The laser emitted by the femtosecond laser passes through a half-wave plate W1 and a Glan prism GL respectively, and is normally incident on the lens group. The lens group expands the light spot and emits it parallelly. Then it is reflected by the mirror group and reflected on the customized slit. The light beam passing through the slit is called the shaped light beam; the shaped light spot is incident on the objective lens OL and is focused by the objective lens OL inside the Corning glass on the sample stage to form a circular light spot; level the sample stage and control its movement to make the laser focus move inside the Corning glass; use the focused circular light spot to prepare the main waveguide with a circular end face, so as to realize the control of the waveguide shape and reduce the waveguide birefringence caused by the asymmetric shape; during the waveguide preparation process, a monitoring system is used to monitor the laser writing process; Step 2: Waveguide stress compensation: After the main waveguide with a circular end face is prepared by using the shaped light beam, keeping the original optical path and beam shaping means unchanged, move the focused laser focus along the direction parallel to the main axis of the main waveguide to process a modified line with an end face diameter smaller than that of the main waveguide; the spatial positions of the modified line and the main waveguide overlap radially, and their lengths are equal, so as to ensure that the stress generated when writing the modified line compensates the existing asymmetric stress in the main waveguide; finally, a new waveguide formed by the combination of the overlapping main waveguide and the modified line is the prepared ultra-low birefringence waveguide; The femtosecond laser in Step 1 emits laser with a wavelength of 515 - 1030 nm, a repetition frequency of 1 MHz, and an output optical power of 0.1 - 5 W; The lens group in Step 1 is composed of a first cylindrical lens CL1 and a second cylindrical lens CL2. The mirror group is composed of a first mirror M1, a second mirror M2, and a third mirror M3. The laser emitted by the femtosecond laser passes through a half-wave plate W1 and a Glan prism GL respectively, and is normally incident on the first cylindrical lens CL1 and the second cylindrical lens CL2. The lens group expands the light spot by 1 - 20 times and emits it parallelly; then it is normally incident on the customized slit through the first mirror M1, the second mirror M2, and the third mirror M3 in sequence.

2. An ultra-low birefringence glass waveguide prepared by femtosecond laser direct writing technology as claimed in claim 1, wherein, The width of the customized slit in Step 1 is 0.2 - 4 mm.

3. An ultra-low birefringence glass waveguide prepared by femtosecond laser direct writing technology as claimed in claim 1, wherein In Step 1, the laser focus is inside the Corning glass, and the circular end face main waveguide is processed at a position 160 - 230 μm away from the surface of the Corning glass. The processing power of the circular end face main waveguide is selected in the range of 300 - 460 mw, and the direct writing speed is 10 - 40 mm / s.

4. An ultra-low birefringence glass waveguide prepared by femtosecond laser direct writing technology as described in claim 1, characterized in that, The monitoring system described in step 2 consists of an illumination module and a monitoring module. The illumination module consists of a white light source and a second convex lens L2. The white light source is placed at the focal point of the second convex lens L2. The second convex lens L2 is placed between the sample stage and the light source. The focal length of the second convex lens L2 is 5 cm, so that the divergent light emitted by the white light source becomes parallel light after passing through the second convex lens L2 and illuminates the entire sample; The monitoring module consists of a fourth mirror M4, a first convex lens L1 and a camera CCD. The three are on the same straight line. The fourth mirror M4 reflects the sample information carried by the illumination light and converges it onto the photosensitive chip of the camera CCD through the first convex lens L1 to achieve imaging.

5. An ultra-low birefringence glass waveguide prepared by femtosecond laser direct writing technology as claimed in claim 1, wherein, The direct writing power for processing the modification line described in step 2 is 20 - 150 mw, and the diameter of the modification line is 20 - 70% of the diameter of the main waveguide of the circular end face; The perpendicular distance between the starting point of the modification line processing and the main axis of the main waveguide of the circular end face varies between 0.05 - 0.5 times the diameter of the main waveguide of the circular end face.

6. Application of an ultra - low birefringence glass waveguide prepared by femtosecond laser direct writing technology as described in claim 1 in the preparation of directional couplers.

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