Saturable absorber devices based on skin effect optical waveguides in glass materials and methods thereof

Abstract

The application discloses a saturable absorption device based on a skin waveguide in a glass material and a method thereof. A skin waveguide is formed by bending a part of a middle portion of a light waveguide in the glass material to a side surface of the glass. A saturable absorption material is arranged on the side surface of the glass. The saturable absorption device is formed by the evanescent field of the light signal leaked from the skin waveguide and the saturable absorption material. The saturable absorption device has the characteristics of low insertion loss and small space occupation. A plurality of skin waveguides with different depths or lengths can be integrated in the glass, the regulation of the saturable absorption characteristics is realized, and the performance is excellent.

Description

Technical Field

[0001] This invention relates to a femtosecond laser processing method in the field of laser processing, and in particular to an on-chip integrated saturable absorber device based on a skin waveguide in glass materials and its method. Background Technology

[0002] In laser systems with ultrashort pulse output, saturable absorbers are the core components for Q-switching and mode-locking, and are also key performance indicators affecting laser performance. Specifically, selecting high-performance saturable absorbers and efficiently coupling them into the laser optical path system can effectively improve the laser's damage threshold, pulse width, and repetition rate modulation depth. Traditionally, compared to coupling methods that directly insert saturable absorbers, coupling methods utilizing the evanescent field of waveguides and matter-matter interaction have many advantages. Because the evanescent field has a weaker light intensity than the central optical field, it can overcome thermal damage caused by high power density, and the saturable absorption effect can be tuned by changing the interaction distance. In previous schemes, evanescent field coupling could be achieved based on D-shaped fibers formed by cutting or tapered fibers formed by fusion, but this damaged the original mechanical structure of the fiber, making these micro / nano fibers more fragile. They typically require additional structural support to protect the micro / nano fibers, adding complexity to fabrication and use, occupying extra space, and hindering the miniaturization of the equipment.

[0003] To address the aforementioned issues, femtosecond laser direct writing technology provides a highly efficient, simple fabrication method that can realize three-dimensional waveguide structures.

[0004] Traditionally, femtosecond laser direct-write waveguides utilize the localized modification caused by the nonlinear absorption of pulsed laser in the focusing region to generate a positive refractive index change, thus forming a waveguide.

[0005] However, when the laser pulse energy is too high and the focused area is on the surface, the heat effect caused by absorption will ablate the glass surface, destroy the uniformity of the waveguide structure, and increase additional transmission loss. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention combines the advantages of femtosecond laser processing, such as high precision, flexibility, and ease of multi-line integration, and proposes a method for on-chip integrated saturable absorber devices in glass materials, as well as a method and application of using femtosecond lasers to directly write saturable absorber devices based on skin waveguides in glass.

[0007] The purpose of this invention is to provide a novel controllable photonic optical path integrated device (including a saturable absorber) that can control the residual stress field distribution in the material after femtosecond laser irradiation of glass, achieving local densification with low or even no stress field influence, and achieving a uniform distribution of waveguide refractive index. The skin waveguide fabricated using this method has the characteristics of low insertion loss and no surface damage; after integration with a saturable absorber, a high saturable modulation depth can be obtained.

[0008] The technical solution adopted in this invention is:

[0009] I. A saturable absorber based on skin waveguide in a glass material:

[0010] A waveguide is provided within the glass material. A section of the waveguide in the middle is bent toward one side of the glass surface to form a skin waveguide. A saturable absorber is provided on the side of the skin waveguide that is close to the glass. The evanescent field of the light signal leaking through the skin waveguide interacts with the saturable absorber to form a saturable absorber.

[0011] The skin waveguide is fabricated by multiple direct writing processes in a glass material using a femtosecond laser.

[0012] The skin waveguide is composed of multiple trajectories arranged closely in parallel along a straight line / curve. Each trajectory is fabricated by direct writing in a glass material using a femtosecond laser, and multiple direct-written trajectories form a waveguide.

[0013] Low-loss waveguides are formed by adjusting the paths, number, and arrangement of individual trajectories in an optical waveguide, with the cross-sectional size of a single trajectory on the submicron or micrometer scale. A single trajectory cannot guide light, so it is stacked and arranged through multiple laser scans. Specifically, it can be directly written to form a cylindrical optical waveguide with a cross-sectional diameter of 10μm that matches the single-mode fiber mode.

[0014] In practice, the lens that focuses the femtosecond laser is fixed on a three-dimensional platform. When the femtosecond laser is writing directly in the glass material, the three-dimensional platform is controlled to move the focal point of the femtosecond laser, thereby controlling the arrangement of single trajectories and the skin depth in the glass material, and thus writing out the skin waveguide.

[0015] The optical waveguide extends through both ends of the glass material, reaching both ends to the two end faces of the glass material and used for interconnection with externally exposed single-mode optical fibers.

[0016] In the described skin waveguide, the waveguides at both ends of the glass are buried inside the glass as deeply buried waveguides, the middle section of the waveguide approaches the glass surface to form a skin waveguide, and two other waveguides with opposite bends connect the skin waveguide and the deeply buried waveguide. The section of the waveguide containing the skin waveguide is called the skin section, and the section of the waveguide containing the deeply buried waveguide is called the deeply buried section.

[0017] The glass material described includes all transparent glass materials, for example: quartz glass, borosilicate glass, porous glass, phosphate glass, tellurite glass, germanate glass, bismuthate glass, fluorosilicate glass, fluorogermanate glass, fluoride glass, etc.

[0018] A saturable absorbing material is integrated on a surface of the glass that is intermediate and close to the skin-effect waveguide, and the saturable absorbing material includes all materials having third-order nonlinear optical saturable absorption properties, including carbon nanotube materials, nanoplasmas, two-dimensional materials, graphene, indium tin oxide, etc., but is not limited to all the foregoing materials having optical nonlinear saturable absorption properties.

[0019] The saturable absorbing material adopts a polyvinyl alcohol / carbon nanotube composite film.

[0020] In specific implementation, skin-effect waveguides with different depths and different lengths are integrated on a single piece of glass, so as to provide saturable absorption parameters with different modulation depths.

[0021] The path of the trajectory in the method is implemented by controlling the movement of a three-dimensional platform, so that a skin-effect optical waveguide with adjustable skin-effect length or depth can be obtained.

[0022] In specific implementation, by using the processing parameters of the trajectories, planning the writing route of each trajectory, and programming to control a three-dimensional precision displacement platform, photonic waveguide devices with various functions can be implemented.

[0023] The femtosecond laser processing parameters use femtosecond lasers with different output wavelengths (e.g., 355, 515, 800, 1030nm, etc.), different pulse widths (40fs–10ps), different repetition frequencies (1kHz–10MHZ), different powers and different polarization outputs, focusing objectives with different numerical apertures (0.4<NA<1.5), different scanning speeds (0.1-50mm / s) and multiple direct writing with different intervals (0.5-1.5μm).

[0024] An optical signal is input from one end of the waveguide, leaks out of the glass through the evanescent field generated at the skin-effect waveguide, and then interacts with the saturable absorbing material on one side surface of the glass to generate a saturable absorption effect. An optical signal with saturable absorption characteristics is output from the other end of the waveguide, whereby an on-chip integrated saturable absorption device is constructed and obtained.

[0025] By preparing the skin-effect waveguides with different depths or lengths, the saturable absorption curve of the saturable absorption device is adjusted accordingly, and then the saturable absorption characteristic parameters of the saturable absorption device are adjusted accordingly.

[0026] The fabrication process of this invention is as follows: By changing the power, pulse width, wavelength, repetition frequency, scanning speed and focusing depth of the laser beam in the sample, as well as the magnification and numerical aperture of the focusing objective, the writing paths of each submicron or micron-level trajectory are planned to achieve a curved skin waveguide with low insertion loss. A saturable absorber thin film is then integrated on the glass surface, and exposed single-mode fiber is used to couple at both ends of the waveguide to form a saturable absorber device, thereby fabricating a Q-switched / mode-locked laser.

[0027] II. Saturable Absorption Method:

[0028] The optical signal is input from one end of the waveguide, and leaks out of the glass through the evanescent field generated at the skin waveguide. It then interacts with the saturable absorber material on one side of the glass, so that the optical signal output from the other end of the waveguide has saturable absorption characteristics, thus realizing the operation of the saturable absorber device.

[0029] This invention can be used in various integrated controllable photonic optical devices, including Q-switched and mode-locked pulsed lasers.

[0030] This invention utilizes a femtosecond laser to directly write a curved waveguide composed of multiple submicron or micron-scale trajectories in glass through multiple scans, so that one section of the waveguide is close to the surface of the glass, which is called a skin waveguide; a saturable absorber is integrated into the skin waveguide, so that the light passing through the waveguide has saturable absorption characteristics. By adjusting the depth or length of the skin waveguide, the saturable absorption curve can be adjusted.

[0031] The principle of this invention is as follows:

[0032] Femtosecond lasers can induce nonlinear absorption effects in glass, creating a positive refractive index change in the modified region. The cross-section of the positive refractive index change induced by a single femtosecond laser scan is on the submicron or micron scale, and a single scan trajectory cannot guide light. By scanning multiple times with a femtosecond laser, multiple trajectories can be stacked to form an optical waveguide with a cross-sectional diameter of approximately 10 μm that matches the single-mode fiber mode. Furthermore, by adjusting the femtosecond laser parameters, the stress field distribution in the glass material can be controlled to achieve low stress or even stress-free distribution. Ultimately, this allows for the writing of a fully positive refractive index optical waveguide at a distance of very close to the glass surface (0-10 μm) without damaging the glass surface.

[0033] Due to the evanescent field effect, a portion of the light transmitted in the waveguide leaks out of the glass and interacts with the saturable absorber material integrated outside the glass, causing the entire device to exhibit optical nonlinear saturable absorber properties, thus providing conditions for the fabrication of Q-switched / mode-locked lasers.

[0034] The femtosecond laser of this invention has adjustable parameters such as wavelength, pulse width, repetition rate, power, polarization, focusing mirror, focusing depth, and scanning speed; the waveguide distance from the surface is adjustable; and the resulting saturable absorption depth and threshold power are adjustable.

[0035] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0036] (1) Compared with traditional processing methods such as nanoimprinting, photolithography, and ion implantation, the present invention is based on femtosecond laser direct writing skin waveguide, which has a simple and efficient processing technology and can realize the processing of three-dimensional structure waveguides.

[0037] (2) Compared with the conventional femtosecond laser direct-write skin waveguide, the present invention overcomes the shortcomings of the traditional laser direct-write skin waveguide, such as uneven refractive index distribution and easy appearance of cavities or cracks on the surface. It enables the femtosecond laser direct-write skin waveguide to be closer to the surface while ensuring low insertion loss. The skin depth and mode leakage are controllable. The evanescent wave in the skin waveguide interacts more strongly with the surface saturable absorbable material, and has excellent saturable modulation depth and saturable threshold power as well as lower insertion loss.

[0038] In summary, the saturable absorber device fabricated by this invention features low insertion loss and small footprint. It can integrate multiple skin waveguides of different depths or lengths into a single piece of glass, thereby achieving control over the saturable absorption characteristics and exhibiting excellent performance. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the fabrication method of femtosecond laser direct-writing skin waveguide in glass material according to the present invention.

[0040] Figure 2 This is a schematic diagram of an on-chip integrated saturable absorber device disclosed in this invention.

[0041] Figure 3 These are actual structural micrographs and near-field mode diagrams of optical waveguides with different skin depths disclosed in this invention.

[0042] Figure 4 The curves show the saturable absorption characteristics of the saturable absorber of the present invention at different waveguide skin depths.

[0043] Figure 5 This invention discloses a Q-switched pulsed laser developed based on the integration of a skin waveguide and a saturable absorber. Detailed Implementation

[0044] The present invention will be further described in detail below with reference to the embodiments, but the implementation of the present invention is not limited thereto.

[0045] The embodiments of the present invention are as follows:

[0046] Example 1

[0047] Figure 1 This is a schematic diagram of the femtosecond laser direct-write "photonic lattice-like" skin waveguide in this embodiment. In this embodiment, Eagle glass is used as the material for the femtosecond laser direct-write "photonic lattice-like" skin waveguide.

[0048] The femtosecond laser direct writing method for "on-chip integrated devices" in this embodiment mainly includes the following three steps:

[0049] Step 1) A femtosecond laser writes a submicron or micron-scale trajectory in a glass material in a single pass;

[0050] The femtosecond laser parameters used were: center wavelength 1030 nm, pulse width 213 fs, repetition rate 1 MHz, and linearly polarized light output. The focusing objective parameters were: an oil-immersed lens with a magnification of 100X and a numerical aperture NA = 1.2; laser power of 10–40 mW; scanning speed of 5 mm / s; and focusing depth of 0–10 μm. Under these femtosecond laser parameters, a single direct writing operation can only form a single trajectory with a positive refractive index change on the sub-micron or micron scale, and this single trajectory has very poor light confinement characteristics and lacks light-guiding properties.

[0051] Step 2) Scan multiple times and write multiple trajectories directly. The trajectories are stacked and arranged according to the set parameters to form a cylindrical skin waveguide that matches the single-mode fiber mode.

[0052] The laser direct writing interval is 0.5-1.5μm, and a total of 40-110 scans are used to form a cylindrical skin waveguide with a diameter of approximately 10μm that matches the mode field of a single-mode fiber. For example... Figure 2 As shown, the skin section in the middle of the waveguide and the two deeply buried sections on both sides are connected by two opposing bending structures with a bending radius of 10 mm. The depth of the skin section ranges from 1 to 10 μm, and the skin length ranges from 0.1 to 5 mm. The insertion loss of the resulting skin waveguide is as low as 1 dB.

[0053] Step 3) Cut the carbon nanotube / polyvinyl alcohol (SWCNT / PVA) saturable absorbent material prepared by solution blending into 5×10 mm pieces. 2 The thin film is then flattened and integrated onto the glass surface of the aforementioned skin waveguide. The exposed single-mode fiber is then aligned with the end face of the waveguide and fixed and encapsulated using UV-cured adhesive with a refractive index matching that of the single-mode silica fiber.

[0054] This embodiment describes the integrated saturable absorber fabricated in Eagle glass using the aforementioned femtosecond laser processing method. Microscopic images and near-field mode diagrams of the actual structures of waveguides at different skin depths are shown below. Figure 3As shown, in regions with relatively deep skin depths (5-10 μm), the skin waveguide shape is a complete circle, and the near-field mode field is a symmetrical circular mode field. In regions with relatively shallow skin waveguide depths (1-5 μm), due to the truncation of the skin waveguide by the air layer, the shape becomes a segmental shape, and the near-field mode field is compressed by the air layer, becoming an elliptical mode field with a shorter upper section and a longer lower section. Skin waveguides with different depths and skin segment lengths of 1 mm can achieve saturable modulation depths of 1-10% and saturable threshold power of 20-200 MW / cm. 2 The test results are as follows Figure 4 As shown.

[0055] Furthermore, an integrated saturable absorber is interconnected with an Er-doped fiber laser to achieve Q-switched pulse output. The Er-doped fiber laser includes a 980nm pump source, a 980 / 1550nm wavelength division multiplexer, Er-doped fiber, fiber coupler, integrated saturable absorber, and polarization controller. Figure 5 The connections for each device are shown in (a). The output waveform of the Q-switched pulsed laser is as follows. Figure 5 As shown in (b).

[0056] The implementation examples demonstrate that this invention enables the direct writing of low-loss, surface-damage-free skin waveguides onto glass surfaces, forming waveguides with adjustable skin depth and length. In the examples above, saturable absorber devices were fabricated, achieving saturable absorption characteristics where the absorptivity varies with incident light intensity. The insertion loss of the waveguide, measured using a 1550nm fiber optic source, was as low as 1dB. Furthermore, the skin waveguides fabricated using the aforementioned femtosecond laser direct writing technology can, in principle, achieve arbitrary skin lengths and depths, and can be applied to any controllable photonic optical path integrated device.

[0057] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the embodiments described above. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A saturable absorber based on a skin waveguide in a glass material, characterized in that: A waveguide is provided within the glass material. A section of the middle part of the waveguide bends toward one side of the glass surface to form a skin waveguide. A saturable absorber is provided on the side of the skin waveguide that is close to the glass. The evanescent field of the light signal leaking through the skin waveguide interacts with the saturable absorber to form a saturable absorber. The skin waveguide consists of a central skin section and two deeply buried sections on both sides. The central skin section and the two deeply buried sections are connected by two opposing bending structures with a bending radius of 10 mm. The depth of the skin section ranges from 1 to 10 μm, and the skin length ranges from 0.1 to 5 mm. The skin waveguide described herein is fabricated by multiple direct writing processes in a glass material using a femtosecond laser. The skin waveguide is composed of multiple parallel trajectories, each of which is fabricated by direct writing in glass material using a femtosecond laser.

2. The saturable absorber based on a skin waveguide in a glass material according to claim 1, characterized in that: The optical waveguide extends through both ends of the glass material, reaching both ends to the two end faces of the glass material and used for interconnection with externally exposed single-mode optical fibers.

3. A saturable absorber based on a skin waveguide in a glass material according to claim 1, characterized in that: The glass has a saturable absorber material integrated on its surface near the skin waveguide in the middle. The saturable absorber material includes all materials with third-order nonlinear optical saturable absorber properties.

4. A saturable absorber based on a skin waveguide in a glass material according to claim 1, characterized in that: The saturable absorbent material is a polyvinyl alcohol / carbon nanotube composite film.

5. A saturable absorber based on a skin waveguide in a glass material according to claim 1, characterized in that: An optical signal is input from one end of the waveguide, leaks out of the glass through the evanescent field generated at the skin waveguide, and then interacts with the saturable absorber material on one side of the glass to produce saturable absorption. The other end of the waveguide outputs an optical signal with saturable absorption characteristics, thereby constructing an on-chip integrated saturable absorber device.

6. A saturable absorber based on a skin waveguide in a glass material according to claim 5, characterized in that: By varying the depth or length of the skin waveguide, the saturable absorption curve of the saturable absorber can be adjusted, thereby adjusting the saturable absorption characteristic parameters of the saturable absorber.

7. A saturable absorption method applied to any one of the saturable absorption devices according to claims 1-6, characterized in that: The optical signal is input from one end of the waveguide, and leaks out of the glass through the evanescent field generated at the skin waveguide. It then interacts with the saturable absorber material on one side of the glass, so that the optical signal output from the other end of the waveguide has saturable absorption characteristics, thus realizing the operation of the saturable absorber device.

8. The application of the saturable absorber according to any one of claims 1-6, characterized in that: Applications in controllable photonic optical circuit devices.

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