JP6190317B2 - 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 in which an undoped YAG single crystal is joined to a Yb: YAG single crystal (3 mm in length) as a laser amplification medium (see, for example, Non-Patent Document 1) ). Excitation light output from the semiconductor laser 109 is input via the lens 110 to a Fabry-Perot resonator in which the bulk rod 101 is installed between the spherical mirrors 106 and 107. A saturable absorber mirror 102 is installed on one side of the Fabry-Perot resonator, and an output mirror 103 of 1% transmission is installed on the opposite side to form a resonator. Dispersion compensation mirrors 104 and 105 are installed between the spherical mirror 107 and the output mirror 103, and a spherical mirror 108 is installed between the spherical mirror 106 and the saturable absorber mirror 102.

  In the oscillation wavelength range (1.02 to 1.08 μm) of the Yb: YAG laser, the group delay dispersion of the Yb: YAG single crystal is a positive value, but the group delay dispersion of the dispersion compensation mirror is a negative value. 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 semiconductor laser 109 (output 4.4 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, Non-Patent Document 1 reports a femtosecond pulse laser having a center wavelength of 1.05 μm, a pulse width of 150 fs, a repetition frequency of 200 MHz, and an output of 43 mW.

S. Uemura and K. Torizuka, “Center-Wavelength-Shifted Passively Mode-Locked Diode-Pumped Ytterbium (Yb): Yttrium Aluminum Garnet (YAG) Laser,” Jpn. J. Appl. Phys. 44 (2005) L361. M. J. F. Digonnet, C. J. Gaete, AND H. J. Shaw, “1.064μm and 1.32-μm Nd: YAG Single Crystal Fiber Lasers,” Journal of Lightwave Technology, Vol. LT-4, No. 4, p. 454-460 (1986). YY Wang, Xiang Peng, M. Alharbi, C. Fourcade Dutin, TD Bradley, F. Gerome, Michael Mielke, Timothy Booth, and F. Benabid1, "Design and fabrication of hollow-core photonic crystal fibers for high-power ultrashort pulse transportation and pulse compression, "Optics Letters Vol. 37, No. 15, p. 3111-3113 (2008).

  However, since the femtosecond pulse laser described above has a Fabry-Perot resonator with an optical path length of 75 cm and is entirely composed of a spatial optical system, the laser oscillator is downsized, for example, the size of the Fabry-Perot resonator is reduced. It was difficult to set it to 30 × 30 cm or less. 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 medium is a single crystal fiber having a shape the outer periphery of the cross-sectional shape approximate to a regular hexagon formed by six arc, the photonic crystal fiber, have a core similar to the cross-sectional shape of the single crystal fiber And optically coupled to the fundamental mode of the single crystal fiber .

  The photonic crystal fiber is composed of a glass film having a thickness of 1 μm or less, and is a waveguide in which the same structure continues in the longitudinal direction, and a six-fold symmetrical hole that functions as a core of the waveguide is formed. Yes. The photonic crystal fiber may have a cross-sectional shape of a two-dimensional kagome lattice.

  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 regular hexagon. The photonic crystal fiber has six-fold symmetric holes that function as the core of the waveguide, and can optically couple both fibers 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 Yb: YAG single crystal fiber concerning one Embodiment of this invention. It is a figure which shows the end surface of the kagome 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. This laser oscillator is composed of a waveguide and a spatial optical system using a Yb: 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 Yb: YAG single crystal fiber 201 is installed between the plane mirror 208 and the spherical mirror 206. A saturable absorber mirror 202 is installed on one side of the Fabry-Perot resonator, and an output mirror 203 of 1% transmission is installed on the opposite side to form a resonator. A dispersion compensating mirror 205 and a kagome 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 saturable absorber mirror 202.

  FIG. 3 shows a Yb: YAG single crystal fiber according to an embodiment of the present invention. The Yb: YAG single crystal fiber 201 has a diameter of 120 μm and a length of 20 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 orientation of the single crystal material is the [111] orientation, and the cross-sectional shape of the fiber is not a circle but a round regular hexagon, that is, the outer periphery of the cross-sectional shape is configured by six arcs.

As shown in FIG. 2, a 1 μm thick SiO ₂ film cladding is formed on the side surface of the fiber to form a Yb: 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 is a shape that approximates a regular hexagon 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.

  An Nd: YAG single crystal fiber, Er: YAG single crystal fiber, or Tm: YAG single crystal fiber grown in the [111] orientation can also be used as the amplification medium of the laser oscillator of this embodiment. A single crystal fiber having a cross-sectional shape similar to a hexagon, such as an Er sapphire single crystal fiber grown in the c-axis direction, 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 is continuous in the longitudinal direction.

  Moreover, the above-mentioned single crystal fiber may not take a shape similar to a hexagon depending on growth conditions. When the Yb: YAG single crystal fiber of this embodiment is manufactured by a normal process, its cross-sectional shape is expressed numerically by six arcs having a curvature of 55% or more of the maximum diameter (a1 to a1 in FIG. 3). a6) is a cross-sectional shape joined by arcs having a curvature of 50% or less of the maximum diameter. It can be easily understood that the numerical expression of the cross-sectional shape is a shape approximated to a regular hexagon, keeping in mind that a circular shape is obtained by connecting six arcs having a curvature of 50% of the maximum diameter. 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 six arcs and approximates a regular hexagon that satisfies the above numerical conditions.

  FIG. 4 shows an end face of a kagome lattice photonic crystal fiber according to an embodiment of the present invention. The kagome lattice photonic crystal fiber 204 is disposed in the optical path between the Yb: YAG single crystal fiber 201 (amplification medium) and the output mirror 203, and has a length of 1 m. The cross section of the kagome lattice photonic crystal fiber 204 has a core structure called a 7-cell inversion cycloid shape (see, for example, Non-Patent Document 3). The kagome lattice photonic crystal fiber 204 is composed of a thin quartz glass film having a thickness of 0.35 μm, and the cross-sectional shape is a two-dimensional kagome lattice. A waveguide is formed by the same structure continuing in the longitudinal direction of the fiber. ing. Seven regular hexagons at the center of the two-dimensional kagome lattice and a lattice of 24 regular triangles surrounding the hexagon form a six-fold symmetric hole, which functions as a waveguide core. The regular hexagon in contact with the holes and the cross section of the glass film shared by the holes are replaced with arcs, and the intensity of the guided light is concentrated here. The spacing between the kagome lattices is 23 μm or less, and the maximum diameter of the core is 79 μm or less. The two-dimensional kagome lattice is formed inside a glass capillary having an outer diameter of 300 μm, and the entire cross-sectional shape is the same external shape as a normal glass fiber.

  As shown in FIG. 4, the kagome lattice photonic crystal fiber 204 has a core similar to the cross-sectional shape of the Yb: YAG single crystal fiber 201 shown in FIG. And the near field pattern of the fundamental waveguide mode of the Yb: YAG single crystal fiber 201 are similar to each other. 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 kagome 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 Yb: 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 flat mirror 208 reflecting 45 °. An output mirror 203 is formed on the end face of the kagome lattice photonic crystal fiber 204, and a resonator is configured between the output mirror 203 and the saturable absorber mirror 202. As described above, since the coupling efficiency between the Yb: YAG single crystal fiber 201 and the kagome lattice photonic crystal fiber 204 can be set sufficiently high, a low threshold operation sufficient for practical use as a laser oscillator can be realized.

  The group delay dispersion of the kagome lattice photonic crystal fiber 204 is negative in the vicinity of the oscillation wavelength and is relatively flat with respect to the wavelength. Accordingly, the dispersion compensation mirror is used so as to compensate the third-order dispersion of the resonator. 205 can be designed easily. These are used to obtain a negative group delay dispersion value that is generally constant throughout the resonator.

  Since the core of the kagome lattice photonic crystal fiber 204 is a hole, the nonlinear optical effect is extremely lower than that of 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 kagome lattice photonic crystal fiber, not only a 7-cell inversion cycloid shape, but also other types of photonic crystal fibers having a 6-fold symmetric core 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 Yb: YAG single crystal fiber 201, the light is guided by the cladding structure in the fiber, so that it diffuses over the entire length of the Yb: YAG single crystal fiber 201. The overlap with the oscillation light is good. Therefore, the excitation light of the semiconductor laser 209 can be used efficiently. Since the excitation light output from the semiconductor laser 209 is condensed according to the size of the end face of the Yb: 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 Yb: 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 1.05 μm, a pulse width of 150 fs, a repetition frequency of 200 MHz, and an output of 43 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. 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, since it required 30 cm × 30 cm or more excluding the semiconductor laser, it can be seen that the size has been greatly reduced.

101 Bulk rod (Yb: YAG single crystal)
102, 202 Saturable absorber mirror 103, 203 Output mirror 104, 105, 205 Dispersion compensation mirror 106-108, 206, 207 Spherical mirror 109, 209 Semiconductor laser 110, 210 Lens 201 Yb: YAG single crystal fiber 204 Kagome lattice photonic Crystal fiber 208 Plane mirror

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 approximating a regular hexagon having an outer periphery of a cross-sectional shape composed of six 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.   2. The single crystal fiber is composed of any one of Yb: YAG single crystal, Nd: YAG single crystal, Er: YAG single crystal, Tm: YAG single crystal, or Er sapphire single crystal. The laser oscillator described in 1.   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 six 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 composed of a glass film having a thickness of 1 μm or less, and is a waveguide in which the same structure continues in the longitudinal direction, and a six-fold symmetrical hole that functions as a core of the waveguide is formed. The laser oscillator according to claim 1, 2 or 3.   The photonic crystal fiber has a two-dimensional kagome lattice in cross-section, and the holes include seven regular hexagons and a lattice of 24 regular triangles surrounding the regular hexagons to form six-fold symmetric holes, The laser oscillator according to claim 4, wherein the regular hexagon in contact with the hole and the cross section of the glass film shared by the hole are replaced with an arc.   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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