JP6190318B2 - Laser oscillator - Google Patents

Description

  The present invention relates to a laser oscillator, and more particularly to a laser oscillator constituting a femtosecond pulse laser used in a spectroscopic analyzer, a high-precision femtosecond laser processing apparatus, and the like.

  The femtosecond pulse laser can output ultrashort pulses of several femtoseconds to several hundred femtoseconds, and is applied to a spectroscopic analyzer that observes a process of a specific chemical reaction in a very short time. A femtosecond pulse laser is also used for processing a semiconductor substrate and the like because it oscillates by compressing energy in a short time and outputs high power light.

FIG. 1 shows the configuration of a conventional femtosecond pulse laser. A laser oscillator configured by a spatial optical system using a bulk rod 101 of Cr ⁴⁺ : YAG single crystal (diameter 3 mm, length 20 mm) as a laser amplification medium (see, for example, Non-Patent Document 1). The excitation light output from the Yb glass fiber laser 109 is input via the lens 110 to a Fabry-Perot resonator in which the bulk rod 101 is installed between the spherical mirror 107 and the dispersion compensation mirror 105. An InGaAs semiconductor saturable absorber mirror 102 is disposed on one side of the Fabry-Perot resonator, and an output mirror 103 of 0.5% transmission is disposed on the opposite side. A dispersion compensation mirror 104 is installed between the dispersion compensation mirror 105 and the output mirror 103, and a dispersion compensation mirror 106 and a spherical mirror 108 are installed between the spherical mirror 107 and the saturable absorber mirror 102.

In the Cr ⁴⁺ : YAG laser oscillation wavelength region (1.48-1.58 μm), the group delay dispersion of the Cr ⁴⁺ : YAG single crystal is a positive value, but the group delay dispersion of the dispersion compensating mirror is a negative value. For this reason, the group delay dispersion in the entire Fabry-Perot resonator becomes a negative value. The pulsed laser light generated by the saturable absorber mirror 102 in the Fabry-Perot resonator by excitation of the Yb glass fiber laser 109 (output 10 W) causes self-phase modulation in the bulk rod 101 and negative dispersion of the Fabry-Perot resonator. As a result, a soliton pulse is stabilized. For example, according to Non-Patent Document 1, a femtosecond pulse laser with a center wavelength of 1.53 μm, a pulse width of 75 fs, a repetition frequency of 130 MHz, and an output of 80 mW is reported and used for high-accuracy infrared light absorption measurement.

A. Alcock, P. Ma, P. Poole, S. Chepurov, A. Czajkowski, J. Bernard, A. Madej, J. Fraser, I. Mitchell, I. Sorokina, and E. Sorokin, "Ultrashort pulse Cr4 +: YAG laser for high precision infrared frequency interval measurements, "Optics Express vol. 13, no. 22, p. 8837-8843 (2005). S. Ishibashi and K. Naganuma, "First Demonstration of Mode-Locked Cr4 +: YAG Single-Crystal Fiber Laser," in Advanced Solid-State Lasers Congress, G. Huber and P. Moulton, eds., OSA Technical Digest (online) (Optical Society of America, 2013), paper ATh3A.7. F. Couny, P. J. Roberts, T. A. Birks, and F. Benabid, "Square-lattice large-pitch hollow-core photonic crystal fiber," Optics Express vol. 16, no. 25, p. 20626-20636 (2008).

  However, in the femtosecond pulse laser described above, the optical path length of the Fabry-Perot resonator is 56 cm from both ends of the bulk rod 101, respectively, and is configured by a spatial optical system. For this reason, it has been difficult to reduce the size of the laser oscillator, for example, to set the size of the Fabry-Perot resonator to 30 × 30 cm or less. In addition, a space for installing the Yb glass fiber laser is required.

  For example, it is possible to increase the number of folding mirrors to reduce the installation area, but the complexity of the apparatus and the difficulty of adjusting the optical path increase. In order to reduce the size of the apparatus, it is desired to develop a laser oscillator with a reduced installation area.

  An object of the present invention is to provide a laser oscillator suitable for constituting a mode-locked laser by reducing the size of the apparatus.

The present invention, in order to achieve the above object, one embodiment, the laser oscillator with a resonator comprising an amplification medium in the optical path, comprising a photonic crystal fiber to replace a portion of the optical path, the amplification The medium is a single crystal fiber having a shape approximate to a regular square or a rectangle whose outer periphery of the cross-sectional shape is composed of four arcs, and the photonic crystal fiber has a core similar to the cross-sectional shape of the single crystal fiber. Yes, and characterized in that it is optically coupled to the fundamental mode of the single crystal fiber.

  The photonic crystal fiber is a waveguide made of a glass film having a thickness of 1 μm or less and having the same structure continuous in the longitudinal direction, and is made of a unit cell having a square cross-sectional shape and functions as a core of the waveguide Holes to be formed are formed.

  According to the present invention, since a part of the resonator conventionally formed by the spatial light path is replaced with a flexible photonic crystal fiber, it is possible to reduce the size of the device.

  The amplification medium of the laser oscillator of the present invention is a waveguide made of a single crystal fiber and has a shape close to a quadrangle. The photonic crystal fiber is constituted by a unit cell having a quadrangular cross-sectional shape, and has a hole having a shape close to a quadrangle that functions as a core of a waveguide, so that both fibers can be optically coupled with high efficiency.

  Since the core of the photonic crystal fiber is a hole, the nonlinear optical effect is extremely low. Also, in the photonic crystal fiber, the wavelength dependence of the group delay dispersion is flat because most of the energy of the oscillation light is guided through the hole portion. Therefore, it is suitable for configuring a mode-locked laser.

It is a figure which shows the structure of the conventional femtosecond pulse laser. It is a figure which shows the structure of the laser oscillator concerning one Embodiment of this invention. It is sectional drawing which shows the ^{Cr4 +} : YAG single crystal fiber ^concerning one Embodiment of this invention. It is a figure which shows the end surface of the square lattice photonic crystal fiber concerning one Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 shows a configuration of a laser oscillator according to an embodiment of the present invention. The laser oscillator is configured by a waveguide and a spatial optical system using a Cr ⁴⁺ : YAG single crystal fiber 201 as a laser amplification medium. Excitation light output from the semiconductor laser 209 is input via a lens 210 to a Fabry-Perot resonator in which a Cr ⁴⁺ : YAG single crystal fiber 201 is installed between the plane mirror 208 and the spherical mirror 206. A semiconductor saturable absorber mirror 202 is provided on one side of the Fabry-Perot resonator, and a 1% transmission output mirror 203 is provided on the opposite side to constitute a resonator. A dispersion compensating mirror 205 and a square lattice photonic crystal fiber 204 are installed between the spherical mirror 206 and the output mirror 203, and a spherical mirror 207 is installed between the plane mirror 208 and the semiconductor saturable absorber mirror 202.

FIG. 3 shows a Cr ⁴⁺ : YAG single crystal fiber according to an embodiment of the present invention. The Cr ⁴⁺ : YAG single crystal fiber 201 has a diameter of 120 μm and a length of 40 mm, and both end faces are provided with a non-reflective coating for the oscillation wavelength (see, for example, Non-Patent Document 2). The growth direction of the single crystal material is 15 ° from [100] to [010], and the cross-sectional shape of the fiber is not a circle but a rounded square, that is, the outer periphery of the cross-sectional shape is composed of four arcs. Yes.

As shown in FIG. 2, a SiO ₂ film cladding having a thickness of 1 μm is formed on the side surface of the fiber to form a Cr ⁴⁺ : YAG single crystal fiber 201. The fiber functions as a waveguide for both excitation light and oscillation light. The mode shape of the fundamental transverse mode to be guided has a shape close to a quadrilateral reflecting the cross-sectional shape. When a bulk crystal is used as an amplification medium like a conventional femtosecond pulse laser, there is a concern about optical axis deviation due to temperature change, but the single crystal fiber of this embodiment has a slight optical axis deviation.

  As an amplification medium of the laser oscillator of this embodiment, Nd: YAG single crystal fiber, Yb: YAG single crystal fiber, Er: YAG single crystal fiber, Tm: grown from [100] to [010] in a 15 ° orientation. A YAG single crystal fiber can also be used. In addition, a single crystal fiber having a cross-sectional shape similar to a square, such as a Ti sapphire single crystal fiber or a Cr forsterite single crystal fiber grown in a direction perpendicular to the c-axis, can also be applied.

  When these single crystal fibers have a diameter (d in FIG. 3) larger than about 1 mm, the spatial light that can pass through has a smaller average mode radius than the guided light, and may not function as a waveguide. Therefore, it is desirable that the amplification medium is a rod-like single crystal fiber in which the same cross section having a maximum diameter of 1 mm or less continues in the longitudinal direction.

Further, the above-described single crystal fiber may not take a shape similar to a square depending on growth conditions. When the Cr ⁴⁺ : YAG single crystal fiber of this embodiment is manufactured by a normal process, its cross-sectional shape is expressed numerically by four arcs having a curvature of 55% or more of the maximum diameter (a1 in FIG. 3). -A4) is a cross-sectional shape joined by arcs having a curvature of 50% or less of the maximum diameter. It is easy to understand that the numerical representation of this cross-sectional shape is a shape that approximates a regular quadrangle or a rectangle, keeping in mind that it is circular if four arcs with a curvature of 50% of the maximum diameter are connected. it can. At least the cross-sectional shape of the single crystal fiber used in the present embodiment is preferably a shape whose outer periphery is composed of four arcs and approximates a regular square or a rectangle that satisfies the above numerical conditions.

FIG. 4 shows an end face of a square lattice photonic crystal fiber according to an embodiment of the present invention. The square lattice photonic crystal fiber 204 is disposed in the optical path between the Cr ⁴⁺ : YAG single crystal fiber 201 (amplification medium) and the output mirror 203, and has a length of 1 m. The square lattice photonic crystal fiber 204 has a core structure called a single cell (see, for example, Non-Patent Document 3). The square lattice photonic crystal fiber 204 is a two-dimensional square lattice with a thin quartz glass film having a thickness of 0.3 μm, and a waveguide is formed by the same structure continuing in the longitudinal direction of the fiber. . Five squares at the center of the two-dimensional square lattice and a lattice of four triangles in contact with this form a hole and function as the core of the waveguide. The intensity of guided light is concentrated in these holes. The spacing of the square lattice is 17 μm and the maximum diameter of the core is 46 μm. The square lattice is formed inside a glass capillary having an outer diameter of 300 μm and an inner diameter of about 240 μm, and the entire cross-sectional shape is the same external shape as a normal glass fiber.

  The cross-sectional shape can be constituted not only by a square but also by a rectangular unit cell including a rectangle and a parallelogram, and the angle at which each side intersects is preferably from 80 ° to 100 °. This is because the shape of the holes formed by the unit cell fusion does not become a core shape suitable for light guiding.

As shown in FIG. 4, the square lattice photonic crystal fiber 204 has a core similar to the cross-sectional shape of the Cr ⁴⁺ : YAG single crystal fiber 201 shown in FIG. The near-field pattern of the wave light is similar to the near-field pattern of the fundamental waveguide mode of the Cr ⁴⁺ : YAG single crystal fiber 201. Therefore, by converting the mode size by the spherical mirror 206 and the dispersion compensating mirror 205, both fibers can be optically coupled with high efficiency. It is considered that the focal position when the emitted light is reflected by the spherical mirror fluctuates due to the thermal lens effect in the laser amplification medium. In the square lattice photonic crystal fiber of this embodiment, since the core diameter is large, the amount of positional deviation is sufficiently smaller than the Rayleigh distance at the focal point, and the influence on optical coupling can be ignored.

The light emitted from the Cr ⁴⁺ : YAG single crystal fiber 201 in the direction opposite to the spherical mirror 206 is condensed on the saturable absorber mirror 202 by the spherical mirror 207 through the plane mirror 208 reflecting 45 °. An output mirror 203 is formed on the end face of the square lattice photonic crystal fiber 204, and a resonator is formed between the output mirror 203 and the saturable absorber mirror 202. As described above, since the coupling efficiency between the Cr ⁴⁺ : YAG single crystal fiber 201 and the square lattice photonic crystal fiber 204 can be set sufficiently large, a low threshold operation sufficient for practical use as a laser oscillator can be realized.

At a wavelength of 1.53 μm, the group delay dispersion value of Cr ⁴⁺ : YAG single crystal fiber 201 is +300 fs ² , whereas the group delay dispersion value of square lattice photonic crystal fiber 204 is −1200 fs ² (for example, non- In combination, the negative dispersion value required for mode-locked oscillation is obtained. Since the dispersion of the square grating photonic crystal fiber 204 is relatively flat with respect to the wavelength, the dispersion compensation mirror 205 can be easily designed to compensate for the third-order dispersion of the resonator. These are used to obtain a negative group delay dispersion value that is generally constant throughout the resonator.

The direction of polarization of the oscillation light is regulated by the crystal orientation of the Cr ⁴⁺ : YAG single crystal fiber 201. However, the square lattice photonic crystal fiber 204 has extremely small birefringence, and the polarization does not rotate within the fiber. It is not a hindrance.

  Since the core of the square lattice photonic crystal fiber 204 is a hole, the nonlinear optical effect is extremely low as compared with a normal quartz glass core fiber. Even when a fiber having a length of 1 m is used in the resonator, the nonlinear optical effect that causes deformation of the oscillation light pulse is not significantly different from that of the spatial optical system.

  As the square lattice photonic crystal fiber, not only a single cell but also a zero cell that deforms a single lattice to be a core instead of merging a plurality of square lattices can be used. The thickness of the glass film forming the lattice of the photonic crystal fiber is desirably 1 μm or less. This is because when the glass film is thicker than 1 μm, a waveguide mode may occur in the film.

The high-power semiconductor laser 209 (wavelength: 0.97 μm, output: 17 W) has a divergent beam and diffuses in a space much larger than the fiber diameter at a distance much shorter than a length of 20 mm. However, as in the present embodiment, when the light is focused on the end face of the Cr ⁴⁺ : YAG single crystal fiber 201, the light is guided by the cladding structure in the fiber, and therefore the total length of the Cr ⁴⁺ : YAG single crystal fiber 201. It is not diffused over a wide range, and the overlap with the oscillation light is good. Therefore, it is possible to efficiently use the excitation light of the semiconductor laser 209 instead of the Yb glass fiber laser. Since the excitation light output from the semiconductor laser 209 is condensed according to the size of the end face of the Cr ⁴⁺ : YAG single crystal fiber 201, the focal length of the lens 210 is relatively short and is 36 mm. However, since the optical path of the oscillation light is bent by the plane mirror 208, the lens 210 can be brought close to the end face of the Cr ⁴⁺ : YAG single crystal fiber 201.

  In the present embodiment, the Fabry-Perot resonator has been described as an example, but the present invention can also be applied to a ring resonator. The output mirror as the output component can be an optical coupler, and the saturable absorber mirror can be a transmissive saturable absorber, and the configuration of the laser oscillator is not limited to the above embodiment.

  When the laser oscillator of this embodiment is used as a femtosecond pulse laser, it is possible to oscillate at a center wavelength of 1.53 μm, a pulse width of 75 fs, a repetition frequency of 130 MHz, and an output of 80 mW, and exhibit the same or better performance as the conventional example. Can do. Whereas the resonator of the conventional example is formed by a spatial light path, in this embodiment, since a flexible photonic crystal fiber is used, the installation area can be reduced. Non-Patent Document 3 shows that a square lattice photonic crystal fiber can be wound with a diameter of 20 cm. The installation area of the laser oscillator of this embodiment is 20 cm × 20 cm or less including the semiconductor laser. In the conventional example, 30 cm × 30 cm or more is required except for the Yb glass fiber laser, and it can be seen that the size has been greatly reduced.

101 Bulk rod (Cr ⁴⁺ : YAG single crystal)
102, 202 Saturable absorber mirror 103, 203 Output mirror 104, 105, 106, 205 Dispersion compensating mirror 107, 108, 206, 207 Spherical mirror 109 Yb glass fiber laser 110, 210 Lens 201 Cr ⁴⁺ : YAG single crystal fiber 204 Square Lattice photonic crystal fiber 208 Plane mirror 209 Semiconductor laser

Claims (6)

In a laser oscillator having a resonator including an amplification medium in an optical path ,
Comprising a photonic crystal fiber that replaces part of the optical path;
The amplification medium is a single crystal fiber having a shape approximate to a regular square or rectangle in which the outer periphery of the cross-sectional shape is constituted by four arcs.
The photonic crystal fiber, the have a core similar to the single crystal fiber cross-sectional shape, a laser oscillator characterized in that it is optically coupled to the fundamental mode of the single crystal fiber. The single crystal fiber is made of Cr ⁴⁺ : YAG single crystal, Nd: YAG single crystal, Yb: YAG single crystal, Er: YAG single crystal, Tm: YAG single crystal, Ti sapphire single crystal, or Cr forsterite single crystal. The laser oscillator according to claim 1, wherein the laser oscillator is configured by any one of the above.   The single crystal fiber has a rod shape in which the same cross section having a maximum diameter of 1 mm or less is continuous in the longitudinal direction, and four arcs having a curvature of 55% or more of the maximum diameter are arcs having a curvature of 50% or less of the maximum diameter. The laser oscillator according to claim 2, wherein the laser oscillator has a cross-sectional shape joined together.   The photonic crystal fiber is a waveguide made of a glass film having a thickness of 1 μm or less and having the same structure continuous in the longitudinal direction, and is made of a unit cell having a square cross-sectional shape and functions as a core of the waveguide The laser oscillator according to claim 1, 2 or 3, wherein holes are formed.   The photonic crystal fiber has an angle of 80 ° to 100 ° intersecting each side of the square unit cell, and the hole has five squares and a lattice of four triangles in contact with the square. The laser oscillator according to claim 4, wherein a hole is formed.   6. The semiconductor laser according to claim 1, further comprising a semiconductor laser, a saturable absorber mirror, and a dispersion compensating mirror, wherein the entire group delay dispersion value of the resonator is a negative value. The laser oscillator described in 1.

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