JP2009295838A - Laser resonator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser resonator capable of stably and selectively only a specific-order transverse mode with respect to higher-order transverse modes having a plurality of ring-shaped intensity distributions of cylindrically symmetric polarized beams. <P>SOLUTION: In the laser resonator capable of oscillating a cylindrically symmetric polarized laser beam, an optical element having one or more annular loss mechanisms for cylindrically symmetric polarized transverse modes other than the specific-order transverse mode is arranged and is caused to selectively oscillate a specific-order cylindrically symmetric polarized transverse mode laser beam. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a laser resonator that selectively and independently generates a higher-order resonator transverse mode among laser light beams having a cylindrically symmetric polarization distribution including radial polarization and azimuth polarization.

  Conventionally well-known polarization states of laser light are linearly polarized light, circularly polarized light or non-polarized light, and the polarization distribution in the cross section of the laser light beam generated from the laser resonator is the same throughout the beam. is there. However, in the case of a cylindrical coordinate system in which the laser optical axis is the z-axis, there may be radial polarization in which linearly polarized light is distributed radially or azimuthally polarized light in a concentric manner as the polarization of the laser light beam. Are known. These are called cylindrically symmetric polarized light because the polarized light is distributed axially symmetrically with respect to the optical axis of the beam.

  The spatial intensity distribution in the beam cross section of the laser light is called a transverse mode and has a certain type. Bessel-Gauss type, Laguerre-Gauss type, and modified Bessel-Gauss type are known as transverse modes of cylindrically symmetric polarized beams, but the basic transverse modes are all cylindrically symmetric singles with no intensity at the center. It becomes a donut shape or a ring shape. Among these, the cylindrically symmetric polarized beam having the Laguerre-Gauss type has a high-order transverse mode.

  A cylindrically symmetric polarized beam having a high-order transverse mode has a plurality of ring-shaped intensity distributions corresponding to the mode order in the beam cross section, and has a phase difference of π radians between adjacent rings. For example, the second-order cylindrically symmetric polarization mode is a beam having a double ring intensity distribution. Among high-order cylindrically symmetric polarized beams with multiple rings, high-order polarized beams can be focused to form a focused spot that is smaller than linearly polarized light with a Gaussian intensity distribution or circularly polarized light. (For example, refer nonpatent literature 1). This fine focused spot property is expected to be applied in the high resolution laser microscope and the optical recording field. In addition, a secondary-polarized beam having a double ring appropriately sets the beam thickness with respect to the aperture of the lens so that the electric field at the focal point disappears, and a minute space surrounded by the electric field is removed. It forms (for example, refer nonpatent literature 2). This is a characteristic realized for the first time by a high-order radial polarization beam, and is expected to be applied to optical tweezers in which fine particles are three-dimensionally confined in this minute space.

  Several devices that generate cylindrically symmetric polarized laser light have been developed, most of which are realized by a mechanism that controls only the polarization state of the laser light, and have selectivity for the spatial intensity distribution in the generated beam cross section. Not done. Therefore, the cylindrically symmetric polarized laser beam realized in the past is a fundamental transverse mode having a single donut-shaped intensity distribution or a multi transverse mode in which a plurality of higher-order modes oscillate simultaneously (for example, Patent Document 1 or Non-Patent Document). 3). That is, the conventionally developed method has a drawback that a high-order transverse mode having a specific order cannot be generated independently.

  In addition, although a method using interference by two orthogonal high-order linearly polarized beams has been devised as one of the methods for generating a high-order polarized beam, it is necessary to adjust the accuracy with respect to the wavelength. (For example, refer nonpatent literature 4).

Y. Kozawa and S. Sato, “Sharper focal spot formed by higher-order radially polarized laser beams”, J. Opt Soc. Am. A, 2007, 24, p.1793 Y. Kozawa and S. Sato, “Focusing property of a double-ring-shaped radially polarized beam”, Opt. Lett., 2006, 31, p.820 T.Moser et al., “Polarization-selective grating mirrors used in the generation of radial polarization”, Appl. Phys. B, 2005, 80, p.707 V. G. Niziev et al., “Generation of inhomogeneouslypolarized laser beams by use of a Sagnacinterferometer”, Appl. Opt. 2006,45, p.8393 JP 2007-227853 A

  Therefore, the present invention provides a laser resonator capable of stably and selectively generating only a specific order of transverse modes of a cylindrically symmetric polarized beam with respect to a high-order transverse mode having a plurality of ring-shaped intensity distributions. With the goal.

  In order to solve the above-described problems, the laser resonator according to the present invention includes one or a plurality of cylindrical symmetric polarized transverse modes other than a specific order in a laser resonator capable of oscillating cylindrically symmetric polarized laser light. An optical element having an annular loss mechanism is arranged, and the laser beam of the specific order cylindrically symmetric polarization transverse mode is selectively oscillated alone.

  According to the present invention, the annular loss mechanism may be any mechanism that generates a large diffraction loss with respect to a transverse mode other than a high-order cylindrically symmetric polarized transverse mode having a specific order. Specifically, by applying absorptive and scattering materials in an annular shape to the reflectors that make up the resonator, or by forming a portion with increased transmittance in the annular shape, etc. A mechanism is obtained.

  The laser resonator according to the present invention may have one or a plurality of optical elements having the annular loss mechanism in at least one of a reflecting mirror and an optical element as constituent elements. The annular loss mechanism may be inserted not only into the reflector of the resonator but also onto a substrate that is transparent to light and inserted into the laser resonator, or inserted into the end face of the laser medium or into the laser resonator. Alternatively, it may be applied on another optical element. As a result, among the cylindrically symmetric polarization modes oscillating in the laser resonator, in the intensity distribution of the transverse mode having a specific order, the portion where the intensity is zero and the node corresponds to the annular loss portion. Only a transverse mode having a specific order can be oscillated without receiving diffraction loss.

  In addition, according to the present invention, by operating an optical element having an annular loss mechanism in the axial direction of the laser resonator, the transverse mode order of the oscillating higher-order cylindrically symmetric polarization mode can be selected.

  The laser resonator according to the present invention has a mechanism for laser oscillation with cylindrically symmetric polarized light. According to the present invention, any laser resonator may be used as long as it has a mechanism capable of selectively oscillating with cylindrically symmetric polarized light. For example, a self-cloning photonic crystal reflective polarizer that can selectively reflect radially polarized light or azimuthally polarized light is used as a reflector of a resonator, a uniaxial crystal having birefringence is inserted into a resonator, or a laser medium A uniaxial crystal can be used.

  Further, according to the present invention, the form of laser oscillation is not limited, and the oscillation form can be continuous wave or pulse oscillation.

  Further, according to the present invention, the oscillation wavelength can be any of the visible region, the infrared region, and the ultraviolet region.

  According to the present invention, the method for exciting the laser medium may be either a side pump or an end pump, or a combination thereof.

  Further, according to the present invention, the inner and outer sides of the reflecting mirror, which is a constituent element, can take any one of a flat surface, a concave surface, and a convex surface.

  According to the present invention, it is possible to stably and selectively generate a high-order transverse mode of a specific order for a high-order transverse mode having a plurality of ring-shaped intensity distributions in a cylindrically symmetric polarized beam.

  FIG. 1 shows a perspective view of an optical element 1 having an annular loss mechanism in a laser resonator according to an embodiment of the present invention. On the self-cloning photonic crystal reflective polarizer 2 that can selectively reflect only the radially polarized light, which is one of the cylindrically symmetric polarized light, the annular loss portion 3 having a greatly reduced reflectance with respect to the radially polarized light is provided. It has been. While the self-cloning photonic crystal reflective polarizer 2 has a concentric groove structure, the annular loss portion 3 is provided with a radial groove structure, thereby reflecting in the photonic crystal structure. The characteristics are changed.

  FIG. 2 is a front view of a laser resonator including the optical element 1 having an annular loss mechanism and the concave reflecting mirror 4. A general laser resonator using the Nd: YAG crystal 5 as a laser medium can be obtained. The laser medium is not limited to the Nd: YAG crystal 5, and other laser media may be used, and the concave reflecting mirror 4 constituting the resonator may be convex or flat.

  In the laser resonator of FIG. 2, a 63 mm Nd: YAG crystal 5 was used, the curvature of the concave reflecting mirror 4 was 500 mm, and the resonator length was 166 mm. The self-cloning photonic crystal reflective polarizer 2 having a concentric pattern was designed to reflect 90% with respect to radial polarization and 20% with respect to azimuth polarization. For the annular loss portion 3, the photonic crystal pattern was a radial pattern, and a circular ring having a thickness of 50 μm and a diameter of 550 μm, which reflects about 20% of the radially polarized light.

  FIG. 3 shows a measurement example of the oscillated laser beam. 3A shows the cross-sectional intensity distribution of the oscillated beam, and FIGS. 3B to 3D show the intensity distribution after transmission through the linearly polarizing plate. The arrows in FIG. 3 indicate the transmission polarization direction of the linearly polarizing plate. FIG. 3A shows that the cross-sectional intensity distribution of the oscillated beam is a double ring-shaped joint distribution. Also, from FIGS. 3B to 3D, four elliptical intensity distributions are obtained along the polarization direction, and this intensity distribution also rotates with the rotation of the polarizing plate. It turns out that it is polarization distribution. From the above, it is shown that this laser oscillation is a single secondary diameter polarization mode.

It is a perspective view which shows the optical element which has the annular | circular shaped loss mechanism provided on the polarization selective reflection type polarizer of the laser resonator of embodiment of this invention. It is a schematic front view of the laser resonator of embodiment of this invention which consists of a polarization selective reflection type polarizer and a concave reflective mirror. (A) Intensity distribution of a transverse section of an oscillated beam, (b) Intensity distribution after transmission through a linearly polarizing plate in the first polarization direction, showing an example of generation of a secondary polarization mode in a laser resonator according to an embodiment of the present invention. (C) Intensity distribution after transmission through the linearly polarizing plate in the second polarization direction, (b) Intensity distribution after transmission through the linearly polarizing plate in the third polarization direction.

Explanation of symbols

1 Optical element having an annular loss mechanism 2 Self-cloning photonic crystal reflective polarizer 3 Annular loss part 4 Concave reflector 5 Nd: YAG crystal

Claims (8)

  In a laser resonator capable of oscillating cylindrically symmetric polarized laser light, an optical element having one or a plurality of annular loss mechanisms for a cylindrically symmetric polarization transverse mode other than a specific order is arranged, and the specific order A laser resonator characterized by selectively oscillating a cylindrically symmetric polarized transverse mode laser beam.   2. The laser resonator according to claim 1, wherein at least one of a reflecting mirror and an optical element which are constituent elements has one or a plurality of optical elements having the annular loss mechanism.   3. The transverse mode order of a high-order cylindrically symmetric polarization mode that oscillates is selected by operating the optical element having the annular loss mechanism in the axial direction of the laser resonator. Laser resonator.   4. The laser resonator according to claim 1, further comprising a mechanism for laser oscillation with cylindrically symmetric polarized light.   5. The laser resonator according to claim 1, wherein the oscillation form is continuous wave or pulse oscillation.   6. The laser resonator according to claim 1, wherein the oscillation wavelength is any one of a visible region, an infrared region, and an ultraviolet region.   7. The laser resonator according to claim 1, wherein a method of exciting the laser medium is either a side pump or an end pump, or a combination thereof. 8. The laser resonator according to claim 1, wherein an inner side and an outer side of the reflecting mirror as a constituent element have a flat, concave, or convex shape.