Single crystal fiber waveguide

Abstract

Provided is a single crystal fiber waveguide capable of suppressing a decrease in laser oscillation efficiency caused by a refractive index distribution due to internal stress. The single crystal fiber waveguide according to the present disclosure comprises a single crystal fiber and a cladding on the outer periphery of the single crystal fiber, the cladding being a laminate film which is formed from a single or a plurality of thin film materials having tensile stress and a single or a plurality of thin film materials having compressive stress and which has a combination of the total film thicknesses of the respective thin film materials, the combination sufficiently suppressing light leakage in a fundamental waveguide mode from the cladding and giving a maximum polarization extinction ratio to the guided transmitted light.

Description

Single Crystal Fiber Waveguide

[0001] The present disclosure relates to single crystal fiber waveguides for use in laser devices and the like.

[0002] Lasers and optical amplifiers utilize light generated by stimulated emission. Because laser light has high energy and high directivity, lasers have been put to practical use in a wide range of fields, including optical communications, medicine, material processing, and material testing. The medium that generates stimulated emission is called the gain medium, and an appropriate gas or solid (single crystal, glass, ceramic) is selected depending on the properties of the required laser light. Single crystals (excluding semiconductors) are often selected when laser oscillation with high-quality femtosecond pulses without pedestals or multi-peaks, high-energy pulses, single-wavelength high-power steady light, etc. is required. A typical single crystal gain medium is yttrium aluminum garnet (hereinafter referred to as YAG), with the chemical formula: Y3Al5O 12 Examples of suitable gain media include yttrium orthovanadate (YVO4), sapphire, etc. These gain media are expected to improve heat dissipation efficiency and oscillation efficiency in semiconductor laser pumping configurations, so it is desirable to confine an optical beam over a long length with a sufficiently small beam diameter. A single crystal fiber waveguide is a single crystal fiber with a waveguide structure capable of optical confinement. From the above perspective, there are high expectations for the commercialization of single crystal fiber waveguides. There are also high expectations for the commercialization of single crystal fiber waveguides as gain media for optical amplifiers that use stimulated emission.

[0003] 1A and 1B are diagrams schematically illustrating the structure of a single crystal fiber waveguide 100 serving as a gain medium according to the prior art, with (a) showing a radial cross section and (b) showing an axial cross section. As shown in Fig. 1, the single crystal fiber waveguide 100 includes a core 101, which is a single crystal fiber, and a cladding 102 formed on the core 101. The core 101 may be manufactured by, for example, laser melt pedestal growth, and the cladding 102 may be formed by, for example, electron beam evaporation. In some cases, the crystal material used for the core 101 may be doped with a metal element such as tetravalent Cr or trivalent Nd.

[0004] The average cross-sectional diameter of the YAG glass was approximately 120 μm. 4+ There have been reports of fabricating a waveguide laser with an external cavity using a single crystal fiber waveguide in which a single crystal fiber of YAG (Yttrium-AlGaAs) is used as the core and a 1 μm-thick silicon dioxide (SiO2) thin film is formed on the core as a cladding by electron beam evaporation. In such a waveguide laser with an external cavity, mode-locked oscillation has been achieved by solid-state laser pumping or direct semiconductor laser pumping (see, for example, Non-Patent Documents 1 and 2).

[0005] On the other hand, it is generally known that when a film is formed on a substrate using a fabrication method including electron beam evaporation, internal stress occurs in the thin film, which in turn causes stress in the substrate (see, for example, Non-Patent Document 3). The internal stress that occurs can be either tensile stress or compressive stress, depending on the materials of the substrate and thin film, the shape of the substrate, the film formation conditions, etc.

[0006] The above-mentioned Cr 4+ Such internal stress also occurs in a single crystal fiber waveguide in which a SiO2 thin film is formed on a YAG single crystal fiber. As a result, regardless of whether the internal stress is tensile or compressive, the Cr core 4+ : A refractive index distribution occurs in the YAG single crystal fiber. 4+ When fundamental transverse mode light is guided through a YAG single crystal fiber, mode conversion to higher modes occurs due to scattering. Furthermore, polarization rotation occurs due to birefringence caused by the refractive index distribution. These phenomena result in a problem of reduced laser oscillation efficiency.

[0007] S.Ishibashi, and K.Naganuma, "Mode-locked operation of Cr4+:YAG single-crystal fiber laser with external cavity," Optics Express, Vol. 22, No. 6, pp. 6764-6771 (2014). S.Ishibashi, "First Mode-Locked Oscillation of Directly Laser-Diode-Pumped Cr4+:YAG Single-Crystal Fiber Laser," The European Conference on Lasers and Electro-Optics, Optica Publishing Group, (2015). Tomonori Aoki, Takashi Inomata, "Optimization of mechanical properties of thin films and optical thin film materials (Special feature: Optical thin films)," O plus E, Vol. 30, No. 8, pp. 835-840 (2008). J.B. Oliver, et al., "Stress compensation in hafnia / silica optical coatings by inclusion of alumina layers," Optics Express, Vol. 20, No. 15, pp. 16596-16610 (2012) Hao Liu, et al., "Stress-compensated anti-reflection coating for high-power laser deposited with IBS SiO2 and ALD Al2O3", Applied Surface Science 476, pp. 521-527 (2019) Robert G. Hunsperger, "Integrated optics", 4th ed., Berlin, Heidelberg: Springer Verlag, pp. 39-43 (1995) H.A. Macleod, "Optical thin films", Nikkan Kogyo Shimbun, pp. 608-611, (1989)

[0008] The present disclosure has been made in view of the above-mentioned problems, and an object of the present disclosure is to provide a single crystal fiber waveguide that can reduce internal stress and suppress a decrease in laser oscillation efficiency caused by a refractive index distribution due to the internal stress.

[0009] In response to the above-described problems, the present disclosure provides a single crystal fiber waveguide having a single crystal fiber as a core, and a clad formed of a plurality of laminated thin films, the clad being composed of a thin film material having a single or multiple tensile stresses and a thin film material having a single or multiple compressive stresses, around the outer periphery of the single crystal fiber, wherein the internal stress generated in the single layer of each thin film material constituting the clad is expressed as σ for the i-th thin film material. i When i is the sum of the thicknesses of all layers made of the i-th thin film material, ξ i is the penetration depth of the fundamental guided mode into the cladding when the i-th thin film material forms a single-layer thin film, λ is the wavelength of light guided through the single crystal fiber waveguide, n is the refractive index of the single crystal fiber, and n i is the refractive index of the ith thin film material constituting the cladding, a is the average diameter of the single crystal fiber waveguide, p is the order of the guided mode, and d of each thin film material constituting the cladding i The present invention provides a single crystal fiber waveguide characterized in that the combination of the above satisfies the following four formulas (Formula A) to (Formula D).

[0010]

[0011] The present disclosure also provides a single crystal fiber waveguide having a single crystal fiber as a core, and a clad around the single crystal fiber, in which multiple thin films are laminated, the clad being made of a thin film material having a single or multiple tensile stresses and a thin film material having a single or multiple compressive stresses, and wherein the thickness of each thin film material constituting the clad is d i The combination of the three formulas (B) to (D) satisfies the three formulas, and among the plurality of different single crystal fiber waveguides for evaluation, the d of each thin film material constituting the cladding that gives the maximum polarization extinction ratio is i The combination of each thin film material that makes up the same cladding i The present invention provides a single crystal fiber waveguide comprising a combination of:

[0012] 1A and 1B are diagrams schematically illustrating the structure of a single crystal fiber waveguide 100 as a gain medium according to the prior art, where (a) is a radial cross-sectional view and (b) is an axial cross-sectional view. 1B are diagrams schematically illustrating the structure of a single crystal fiber waveguide 200 according to first and second embodiments of the present disclosure, where (a) is a radial cross-sectional view and (b) is an axial cross-sectional view. 1C are diagrams schematically illustrating the structure of a single crystal fiber waveguide 300 according to first and second embodiments of the present disclosure, where (a) is a radial cross-sectional view and (b) is an axial cross-sectional view. 1D are diagrams schematically illustrating the configuration of a polarization extinction ratio device 400 used to determine the structure of a single crystal fiber waveguide according to a second embodiment of the present disclosure.

[0013] Various embodiments of the present disclosure will be described in detail below with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant description may be omitted. Materials and numerical values ​​are for illustrative purposes only and are not intended to limit the technical scope of the present disclosure. The following description is an example, and some configurations may be omitted or modified, or additional configurations may be added, as long as they do not deviate from the gist of one embodiment of the present disclosure.

[0014] First Embodiment The structure of a single crystal fiber waveguide 200 according to the present disclosure is shown schematically in Figure 2. Figure 2(a) shows the radial cross-sectional structure of this single crystal fiber waveguide 200, and Figure 2(b) shows the axial cross-sectional structure of the same waveguide. 4+ A thin aluminum oxide (Al2O3) film 202 having a thickness of d1 μm is formed on the outer periphery of a YAG single crystal 201 by electron beam evaporation, and a thin SiO2 film 203 having a thickness of d2 μm is further formed on the outer periphery of that.

[0015] The thicknesses d1 and d2 of the thin film satisfy the following two conditions:

[0016] (1) The tensile stress of the Al2O3 thin film and the compressive stress of the SiO2 thin film are offset, reducing the force applied to the single crystal fiber from the overall stress of the laminated film. The internal stresses of the Al2O3 thin film and the SiO2 thin film fabricated on the planar substrate are measured as the basis for determining d1 and d2. The deposition conditions for these two types of thin films, such as the temperature of the substrate, the atmosphere (oxygen concentration, pressure, etc.), the deposition rate, and the post-deposition heat treatment method, are the same as those used to fabricate the actual single crystal fiber waveguide. The internal stress can be measured using, for example, the disk method, the cantilever beam method, or a crystal structure analysis method (e.g., X-ray diffraction method) (see, for example, Non-Patent Document 3). Furthermore, values ​​determined by prior research published in literature may be used as the internal stress for determining d1 and d2. If the internal stress values ​​for the Al2O3 thin film and the SiO2 thin film on the planar substrate are σ1 and σ2, respectively, the average stress of the entire laminated film is the left side of Equation 1 (see, for example, Non-Patent Document 4). According to the standard of Non-Patent Document 5, the thicknesses d1 and d2 are set so that the absolute value of the left side of (Equation 1) is 100 MPa or less.

[0017]

[0018] (2) In order to sufficiently reduce the transmission loss of a single crystal fiber waveguide, it is necessary to suppress the leakage of guided light from the cladding. Since the cross section of a single crystal fiber is roughly rectangular, it is acceptable to evaluate the penetration depth of guided light using the Marcatili method, a guided mode calculation method for rectangular waveguides. When the core is square and the cladding refractive index is uniform, the calculated penetration depth ξ of the fundamental guided mode into the cladding is expressed by the following formula (see, for example, Non-Patent Document 6):

[0019]

[0020] λ is the oscillation wavelength, n and n c are the refractive index of the core and the refractive index of the cladding at the oscillation wavelength λ, respectively. a is the length of one side of the core, which is the average diameter of the single crystal fiber waveguide. p is the order of the guided mode, and p = 1 for the fundamental guided mode.

[0021] The refractive indices of the Al2O3 thin film and the SiO2 thin film at an oscillation wavelength of 1.5 μm are 1.56 and 1.445, respectively (see, for example, Non-Patent Document 7). When the cladding is formed of a single thin film material, the penetration depth ξ of the fundamental guided mode into the cladding is calculated from the refractive index of 1.81 of a YAG single crystal at an oscillation wavelength of 1.5 μm, and is 0.26 and 0.22 for the Al2O3 thin film and the SiO2 thin film, respectively, as calculated from (Equation 2) and (Equation 3).

[0022] When calculating the penetration depth of a two-layer cladding into the outer cladding, the average refractive index calculated by treating the inner cladding and core as a single unit is used as the refractive index of the core. The average diameter of the entire core and the inner cladding is used as the length of one side of the core. In this embodiment, the cladding is sufficiently thin compared to the average diameter of the core, so there is no problem in using the refractive index of the core and the average diameter of the core as the average refractive index and the average diameter of the entire core, respectively. Therefore, the penetration depth ξ into the outer cladding is the same as when the cladding is made of a single-layer film of that thin film material.

[0023] The logarithmic intensity ratio of the fundamental guided mode outside the core to the cladding is calculated from the left side of Equation 4. To sufficiently suppress light leakage of the fundamental guided mode from the cladding, the thicknesses d1 and d2 are set so that the value of the left side is 3 or more.

[0024]

[0025] A laminated film that satisfies the above conditions (1) and (2) is used for the cladding. Since the condition (1) is satisfied, the force applied to the core of the single crystal fiber waveguide from the cladding is lower than in the conventional example, and the refractive index difference of the refractive index distribution in the core is reduced. Cr 4+A mode-locked laser with an external cavity using a YAG single crystal fiber waveguide is described in Non-Patent Document 1. In this case, a silica glass rod is used to compensate for intracavity group velocity dispersion, enabling the generation of ultrashort pulses through dispersion compensation. The input and output faces of the glass rod are positioned at a Brewster angle relative to the optical axis of the oscillating light. Therefore, if polarization rotation of the oscillating light occurs during transmission through the single crystal fiber waveguide, losses occur in the glass rod, resulting in reduced oscillation efficiency. The refractive index profile of the core caused by internal stress in the cladding is accompanied by birefringence, which causes polarization rotation. The single crystal fiber waveguide disclosed herein has reduced core birefringence compared to conventional examples, resulting in reduced polarization rotation of the oscillating light and improved laser oscillation efficiency. Furthermore, since condition (2) is satisfied, the transmission loss of the single crystal fiber waveguide is sufficiently low, and does not interfere with the improved laser oscillation efficiency achieved by satisfying condition (1).

[0026] When the cladding is made up of two or more types of thin film materials, and when the number of layers of each material is one or more, (Equation 1) to (Equation 4) can be expanded to the following:

[0027]

[0028] where σ i is the stress of the i-th thin film material, d i is the sum of the thicknesses of all layers of the i-th thin film material, ξ i is the penetration depth of the fundamental guided mode into the cladding when the i-th thin film material forms a single-layer thin film.

[0029] Figure 3 shows an example of a single crystal fiber waveguide 300 of the present disclosure, which uses three types of thin film materials. Figure 3(a) shows the radial cross-sectional structure of this single crystal fiber waveguide 300, and Figure 3(b) shows the axial cross-sectional structure of the same waveguide. 4+ : The YAG single crystal 301 has a laminated film formed by electron beam evaporation on its outer periphery. 1a An Al2O3 thin film 302 having a thickness of d2 μm is attached to the periphery of the Al2O3 thin film 302, and an SiO2 thin film 303 having a thickness of d2 μm is attached to the periphery of the Al2O3 thin film 302. 1b The Al2O3 thin film 304 has a thickness of d3 μm, and a magnesium oxide (MgO) thin film 305 with a thickness of d3 μm is disposed around the periphery of the Al2O3 thin film 304.1a and d 1b In this case, d1, d2, and d3 are determined so as to satisfy the four equations (Equation 5) to (Equation 8).

[0030] (Second embodiment) In the first embodiment, the thickness of each layer was set to reduce the average internal stress of the entire laminated film based on the measured internal stress of the single-layer film on the flat substrate. However, because the laminated film is fabricated on the convex surface of the outer periphery of the single crystal fiber, the internal stress of the thin film formed on the flat substrate does not necessarily match the internal stress of the thin film on the outer periphery of the single crystal fiber. It is also known that the internal stress of a thin film changes depending on the film formation conditions (for example, Non-Patent Document 3).

[0031] Therefore, laser light is guided through a single crystal fiber waveguide, and the polarization extinction ratio of the transmitted light is measured and used as an alternative index for stress measurement to determine film thickness. As mentioned above, the refractive index distribution of the core due to internal stress in the cladding is accompanied by birefringence, so evaluating the core birefringence using the polarization extinction ratio provides an index that reflects the stress in the cladding. Figure 4 shows a polarization extinction ratio measurement system 400. A linearly polarized beam 405 from a linearly polarized laser light source 401 is incident on a λ / 4 wave plate 402 and then incident on a single crystal fiber waveguide 406 in the desired polarization direction. The beam passes through a polarizer 403 and is measured by a power meter 404.

[0032] In a single crystal fiber waveguide for evaluation having the same structure as the single crystal fiber waveguide to be actually used, the thickness d1 of the Al2O3 thin film and the thickness d2 of the SiO2 thin film are used as parameters, and a plurality of different waveguides that satisfy Equation 4 and for each combination of d1 and d2 are fabricated, and the polarization extinction ratio of each is measured. The combination of d1 and d2 that gives the maximum polarization extinction ratio as a result of the above measurements is used as the combination of thicknesses of each cladding layer of the single crystal fiber waveguide 200 to be actually used.

[0033] Similarly, when the cladding has two or more types of materials and when the number of layers of each material is one or more, a plurality of single crystal fiber waveguides for evaluation that satisfy the three formulas (6) to (8) and are different from one another are fabricated for the combination of the thickness of each cladding layer, and the polarization extinction ratio of each is measured. i The combination of each thin film material constituting the cladding in the single crystal fiber waveguide to be actually used is i Used in combination with.

[0034] In both the first and second embodiments, the thin film may be formed by other film forming methods such as ion-assisted deposition, sputtering, ion plating, or a sol-gel method in addition to electron beam deposition.

[0035] In both the first and second embodiments, YAG is used as the Yb 3+ , Nd 3+ , Er 3+ , Tm 3+ , Ho 3+ Single crystals containing either of the above or Cr 4+ Additive forsterite single crystal or Ti 3+ Doped sapphire single crystal or Yb 3+ , Nd 3+ , Er 3+ , Tm 3+ , Ho 3+ Lutetium aluminum garnet (hereinafter referred to as LuAG), chemical formula: LuAlO 12 ), yttrium oxide (Y2O3), lutetium oxide (Lu2O3), scandium oxide (Sc2O3), or YVO4 single crystal may be used as the core of the single crystal fiber waveguide.

[0036] Furthermore, in both the first and second embodiments, other thin film materials may be used in place of Al2O3, SiO2, and MgO, such as Y2O3, silicon monoxide (SiO), and hafnium oxide (HfO2).

[0037] Furthermore, in the case of a laser using a YAG medium, if a highly divergent multi-transverse mode semiconductor laser is used for excitation, it is necessary to use a thin film of a material with a refractive index of 1.77 or less at the excitation wavelength. Since the refractive index of the waveguide core is 1.81, if the refractive index is 1.77 or less, the NA of the waveguide will be 0.37 or more, and the highly divergent excitation light can be confined within the waveguide.

[0038] As described above, the single crystal fiber waveguide according to the present disclosure is expected to have high oscillation efficiency when used as a gain medium for a laser because the refractive index difference within the core caused by the internal stress of the outer thin film of the fiber is reduced compared to conventional techniques.Furthermore, due to the same physical mechanism, it is expected to have high amplification efficiency when used as a gain medium for an optical amplifier.

Claims

1. a single crystal fiber as a core; On the outer periphery of the single crystal fiber, a thin film material having single or multiple tensile stresses; Consisting of thin film materials with single or multiple compressive stresses, a clad formed by stacking a plurality of thin films; 1. A single crystal fiber waveguide comprising: The internal stress generated in the single layer of each thin film material constituting the cladding is expressed as σ for the i-th thin film material. i When d i is the sum of the thicknesses of all layers made of the i-th thin film material, ξ i is the penetration depth of the fundamental guided mode into the cladding when the i-th thin film material forms a single layer film, λ is the wavelength of light guided through the single crystal fiber waveguide, n is the refractive index of the single crystal fiber at that wavelength, and n i is the refractive index at the wavelength of the i-th thin film material constituting the cladding, a is the average diameter of the single crystal fiber waveguide, p is the order of the waveguide mode, and d of each thin film material constituting the cladding i The above single crystal fiber waveguide is characterized in that the combination of the above satisfies the following four formulas (Formula A) to (Formula D). [Equation 1]

2. a single crystal fiber as a core; On the outer periphery of the single crystal fiber, a thin film material having single or multiple tensile stresses; Consisting of thin film materials with single or multiple compressive stresses, a clad formed by stacking a plurality of thin films; 1. A single crystal fiber waveguide comprising: d of each thin film material constituting the clad i The combination of the above satisfies the three formulas (B) to (D) and provides the maximum polarization extinction ratio among a plurality of different single crystal fiber waveguides for evaluation. i The same combination of thin film materials constituting the clad as i The single crystal fiber waveguide comprises a combination of: [Equation 2]

3. the core of the single crystal fiber waveguide is Yttrium aluminum garnet (YAG) with Cr 4+ , Yb 3+ , Nd 3+ , Er 3+ , Tm 3+ , Ho 3+ A single crystal to which one of the following has been added: Or Cr 4+ Doped forsterite single crystals, Or Ti 3+ Doped sapphire single crystal, or lutetium aluminum garnet (LuAG), yttrium oxide (Y 2 O 3 ), lutetium oxide (Lu 2 O 3 ), scandium oxide (Sc 2 O 3 ), yttrium orthovanadate (YVO 4 ) to Yb 3+ , Nd 3+ , Er 3+ , Tm 3+ , Ho 3+ A single crystal to which one of the following has been added:

3. The single crystal fiber waveguide according to claim 1, wherein the waveguide is formed of one of the following:

4. The thin film material is silicon dioxide (SiO 2 ), aluminum oxide (Al 2 O 3 ), magnesium oxide (MgO), Y 2 O 3 , silicon monoxide (SiO), hafnium oxide (HfO 2 3. The single crystal fiber waveguide according to claim 1, wherein either one of the following is used:

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