JP5347127B2 - Diode-pumped laser device using anisotropic laser crystal - Google Patents

Description

The present invention relates to a diode-pumped laser device using an anisotropic laser crystal.
More particularly, the present invention can improve the symmetry of the spot focused in the anisotropic laser crystal when the anisotropic laser crystal is pumped by radiation having a narrow striped output beam, Utilizing anisotropic laser crystals that can improve the efficiency of laser systems, especially when pumping solid state lasers in vertical pumping form using striped laser diodes, laser diode bars and laser diode arrays, etc. The present invention relates to a diode-pumped laser device.

  Recently, the field of solid-state laser systems that are optically pumped by laser diodes, laser diode bars, laser diode arrays, etc. has been greatly developed.

  Laser diodes are suitable as pumping light sources because they are small in size, useful for practical use, and do not cause heating problems associated with light sources such as flash lamps. Furthermore, since the laser diode is integrated, it can be used efficiently in a vertical pumping structure.

In order to generate a laser radiation transverse mode TEM ₀₀ with high efficiency, as in existing laser devices pumped with a laser diode, if the laser diode beam and the laser beam oscillating in the resonator overlap well, A vertical pumping system is more effective than a horizontal pumping system.

In other words, maximum efficiency is obtained with a diode-pumped laser system when the mode volume of the laser diode in the laser crystal matches the TEM ₀₀ mode of the laser resonator.

It is difficult for many laser diodes to satisfy these conditions.
This problem is related to the fact that laser diodes (and especially high power laser diodes) have a long rectangular beam output region and an asymmetrical form in which the output beam width is much larger than the height. There is.

Furthermore, the brightness distribution in the two vertical (orthogonal) directions of the laser diode beam expressed by the beam quality factor M ² is very asymmetric.

  For a typical high power laser diode, such as a narrow striped laser diode, the divergence angle in the narrow height direction (fast axis) is even greater than the divergence angle in the wide width direction (slow axis).

  However, because the width is much larger than the height, the brightness at the height is still more than 10 times greater than the brightness at the width.

  Therefore, considerable effort is required to improve the output beam of a laser diode having a very asymmetric distribution within the laser crystal and to match the resonator beam having a symmetric distribution.

  In order to make the output beam of the laser diode coincide with the resonator beam, various optical elements such as a prism and a lens are employed to change the aspect ratio of the laser diode pumping beam inside the laser crystal.

  As one such approach (approach) method, a laser diode beam is first passed through an aspherical lens so that two different axes (fast axis and slow axis) are parallel.

  In a standard laser resonator that does not use a cylinder-shaped optical component, the laser mode inside the resonator is somewhat round.

  Therefore, in order to achieve mode matching between the laser beam and the pump beam, the pump beam must be converged to be a circular spot inside the gain medium (the material from which the light is amplified).

Generally, when a lens is placed in the optical path of a laser beam having a beam quality factor M ² , the passed beam is converged with a radius size w ₀ as shown in Equation 1 below.

Where λ represents the wavelength of the pump beam, f is the length of the focal point of the _lens , and w _lens is the radial size of the beam that has passed through the lens. It was assumed that the radius of curvature of the beam that passed through the lens was much larger than the focal length.
Equation 1 shows that the beam radius after passing through a lens with a given focal length f from a fixed wavelength is proportional to M ² / w _lens .

Therefore, in order to obtain a round shaped spot after passing through the spherical converging lens, the pump beam must be expanded in the slow direction by the ratio of beam quality factors, M ² _slow / M ² _fast .

Such magnification is usually obtained using a Kaplan or Galileo type cylinder magnifier. In the case of a laser diode with a narrow stripe pattern having a large area of about M ² _slow / M ² _fast ≈15, the lens beam that has passed through the lens is strongly expanded in the slow direction. This uses a large diameter lens because the size of the pump beam increases.

  When converging the pump beam to a small area, a very high pump intensity can be obtained with a gain medium due to the high brightness of the diode. Such high pump strength is particularly required in devices similar to the three levels in order to more easily overcome the transparent strength.

  Several types of anisotropic laser crystals operating in a device similar to the three levels, such as Yb: KGW and Yb: KYW crystals, have various wavelength absorption bands depending on the polarization. Absorption is maximized when the vector direction of the electric field is parallel to the a-axis of the laser crystal.

  In order to reduce the loss due to reflection at the end face of the crystal, the laser medium is cut at a Brewster angle (polarization angle). In this case, when the pump beam is incident on the laser crystal, the size of the pump beam increases in the incident plane inside the laser crystal due to the refractive index of the laser crystal.

  In the case of a laser crystal having a large refractive index, the size of the laser beam and the asymmetry of the beam are seriously increased. Large area laser diodes mostly have an output polarized in TE (Transverse Electric) mode with a slow axial polarization vector.

  Therefore, for effective pumping of the anisotropic laser crystal, the polarization direction of the laser diode must be aligned with the direction in which the absorption of the laser crystal is maximized and placed in the Brewster angle incident plane.

  The brightness of the laser diode beam in the slow axis direction is basically smaller than the brightness in the fast axis direction. This causes severe asymmetries in the laser diode beam when the beam is focused inside the laser crystal, causing a mode mismatch with the laser resonator.

  For example, according to Non-Patent Document 1, the pump beam of a laser diode is asymmetrically converged in air at 31 μm × 165 μm, but this increases 1.5 times in the laser crystal and mode mismatch with the laser resonator Will increase.

Despite this, it is beneficial to develop diode-pumped lasers that can utilize anisotropic laser crystals and operate with high efficiency and high power in the TEM ₀₀ mode where diffraction is nearly limited. It is.

  This is because a substance having anisotropic characteristics has characteristics suitable for a specific application. Compared to a medium doped with neodymium (Nd), anisotropic laser crystals containing ytterbium (Yb) such as Yb: KGW and Yb: KYW have a large bandwidth and a long lifetime in the upper state.

Such characteristics are important when designing a mode-locked laser with femtosecond pulse width and high average power, providing an adjustable variable.
In addition, several properties of anisotropic ytterbium doped crystals are attractive for diode pumping.

In other words, the diode pump wavelength from 940 nm to 980 nm has a very high absorption coefficient, and the quantum defect between the pump beam wavelength and the oscillation beam wavelength is low. Can be combined.
A. Shirakawa, K. Takaichi, H. Jagi et al., Optics express, 11,2911 (2003)

  Anisotropic laser crystals such as Yb: KGW and Yb: KYW are used in certain lasers that use higher pump power. However, the strong asymmetry of the pump beam limits the power and efficiency of the vertical pumping structure.

  On the other hand, there is a need for a laser with high efficiency. If the efficiency of the laser is low, a laser diode with a higher pumping power is required to obtain the desired laser power.

  In particular, when the output is increased with a narrow striped laser diode light source, the diode junction temperature increases and the life of the laser diode light source decreases. This is unacceptable for applications requiring a long lifetime.

  In addition, low efficiency lasers must additionally use a pump light source to obtain the desired laser power. This is unacceptable for cost-sensitive applications and applications that are sensitive to complexity.

  Although anisotropic laser crystals are used, there is a need for high efficiency and high power lasers that provide high quality beams with a wide range of pump power. There is then a need for a diode-pumped solid state laser with reduced thermal birefringence.

The object of the present invention is to reconstruct the intensity distribution in the abscissa direction inside the laser crystal when using pumping light sources with very asymmetrical diffractive properties such as laser diodes, diode bars and diode arrays. The beam focused on has a nearly circular spot with an absorption length, so that it operates in TEM ₀₀ mode, generating polarized output but very efficient, and pumping light emitted from the laser diode. An object of the present invention is to provide a diode-pumped laser device using an anisotropic laser crystal that can be focused in a laser medium at high density.

In order to achieve the object, according to the present invention,
Along the optical axis,
A pump radiation source for generating a pump beam having a smaller asymmetric cross section and brightness in a second direction (slow direction) that is smaller in a first direction (early direction) and perpendicular to the first direction; An aspheric lens that collimates the pump beam output from the first direction;
A polarization rotation device that rotates the polarization direction of the pump beam that has passed through the aspheric lens by 90 ° in order to change the polarization direction from the TE mode to the TM mode;
A first cylinder lens that enlarges the size of the pump beam that has passed through the polarization rotator in the second direction;
A second cylinder lens that collimates the pump beam that has passed through the first cylinder lens in the second direction;
A converging lens;
By reconstructing the intensity distribution in the direction of the horizontal axis inside the anisotropic laser crystal, the finally focused beam has an almost circular spot with an absorption length, so that it operates in the TEM ₀₀ mode. Formed by a tungstate medium doped with Yb ³⁺ ions, having characteristics and laser characteristics, having first and second axes crystallographically orthogonal to each other, these axes being the polarization of the pump beam And anisotropic laser crystals having different absorptions depending on each other, and
Making the asymmetric pump beam output from the pump radiation source into parallel light by the aspheric lens, the first cylinder lens, and the second cylinder lens;
The converging lens converges a pump beam collimated by the aspheric lens, the first cylinder lens, and the second cylinder lens on a surface of a beam entrance of the anisotropic laser crystal,
A first dichroic resonator mirror is disposed between the converging lens and a beam entrance of the anisotropic laser crystal;
A second dichroic resonator mirror is disposed at the beam exit of the anisotropic laser crystal, laser light is output to the first dichroic resonator mirror, and the reflected light is output to the converging lens. A first laser resonator mirror that is incident on the beam entrance of the anisotropic laser crystal together with a pump beam from
It said second outputs laser light to dichroic resonator mirror, is incident on the reflected light beam outlet of the anisotropic laser crystal, and a second laser resonator mirror is arranged,
Pumping the anisotropic laser crystal using a pump beam output from the pump radiation source;
A laser device is provided.

As a preferred embodiment, in the present invention, the anisotropic laser crystal is cut at a Brewster angle.

As a more preferred embodiment, in the present invention, the direction of polarization of the pump beam output from the pump radiation source is set to be anisotropic laser so that the anisotropic laser crystal can absorb the pump beam to the maximum extent. Located along the a-axis of the crystal.

  Furthermore, in the present invention, the anisotropic laser crystal is cut at a beam entrance and exit at a Brewster angle, and the Brewster angle increases according to the ratio of the refractive index of the laser crystal to the air refractive index.

Preferably, in the present invention, the anisotropic laser crystal is a Yb: KYW or Yb: KGW crystal that is a tungstate medium to which Yb ³⁺ ions are added.

Further preferably, in the present invention, the polarization rotation device is a half-wave plate.

Further preferably, in the present invention, the pump radiation source is any one selected from a laser diode, a laser diode bar, and a laser diode array in which each emitter emitting a pump beam has a linear form.

According to the diode-pumped laser apparatus using the anisotropic laser crystal according to the present invention, when using a pump light source having a highly asymmetrical diffractive property such as a laser diode, a diode bar and a diode array, the laser crystal Reconstructs the intensity distribution in the horizontal direction inside, and the finally focused beam has a nearly circular spot with absorption length, so that it can operate in TEM ₀₀ mode and generate polarized output The efficiency is very high and the pumping beam emitted from the laser diode can be focused into the laser medium at high density.

  Hereinafter, a diode-pumped laser device using an anisotropic laser crystal according to the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a diagram showing a cross-sectional pattern and an axial direction of an asymmetric beam having a narrow stripe pattern 10 generated from a laser diode.

In a commercial watt class laser diode, the ratio of width (W) to height (H) is 100-1. The divergence angle on the slow axis in the width (W) direction is about 10 °, and the beam quality factor M ² is about 15.

On the other hand, the divergence angle at the fast axis in the height (H) direction is about 30 °, but the beam quality factor M ² is about 1. Thus, the output of the laser beam is mostly polarized along the slow axis direction.

  FIG. 2 is a diagram showing the axis of the laser active material made of the anisotropic laser crystal 11 such as Yb: KYW and the direction of the Brewster angle (θ) on the incident plane.

  The output aperture of the laser diode 14 is placed along the a-axis 13 of the anisotropic laser crystal 11 so that the slow axis direction, ie the polarization direction 12, is maximized in order to maximize absorption in the anisotropic laser crystal 11. Due to the low brightness of the laser diode 14 in the slow axis direction, the pump beam size in the slow axis direction mostly exceeds the beam size in the fast axis direction.

The anisotropic laser crystal 11 serves as a pump radiation source 14, for example, to eliminate reflection loss when the pump beam emitted from a laser diode and the laser beam of the laser resonator mirror 19 are TM polarized light. Cut at Brewster angle (θ).

  Then, in order to minimize the loss with the anisotropic laser crystal 11 cut at the Brewster angle (θ), the polarization vector of the laser diode 14 is placed on the incident plane. Therefore, in the slow axis direction, the size of the beam is additionally increased due to refraction at the Brewster surface.

  This is represented by a pump beam having a rectangular cross section inside the laser crystal 11.

The anisotropic laser crystal 11 is a Yb: KYW or Yb: KGW crystal which is a tungstate medium to which Yb ³⁺ ions are added.

The tungstate medium doped with Yb ³⁺ ions has received much attention in recent years because it has spectroscopic characteristics and laser characteristics used in diode-pumped solid-state lasers ( For example, Non-Patent Document 2: [NVKuleshov, AALagatsky, AVPodlipensky, and VPMikhailov, and G. Huber, “Pulsed laser operation of Yb-doped KY (WO4) 2 and KGd (Wo4) 2, Optics Letters Vol.22, No. 17, pp. 1317-13 (1997)].

  The anisotropic laser crystal 11 has a monoclinic C2 / c structure. The unit cell parameters for Yb: KGW are a = 0.08095 nm, b = 1.043 nm, c = 0.7588, and for Yb: KYW, a = 0.805 nm, b = 1.35 nm, c = 0.754 nm.

  The absorption and emission spectra of Yb: KYW, which is an anisotropic laser crystal 11 doped with 5% Yb ions as atomic units, at room temperature are as shown in FIG. 5 (Non-Patent Document 2).

  As shown in the figure, when the polarization direction is parallel to the a-axis 13, that is, E‖a, it can be confirmed that absorption is strongest at a wavelength of 981.2 nm, and the wavelength most suitable for laser oscillation is 1025 nm. The width of the emission line is about 16 nm.

  The anisotropic laser crystal is made by an improved Czochralski method. The photograph of FIG. 6 is an example thereof, and shows a Yb: KYW laser crystal having a thickness of 3 mm doped with Yb ions at an atomic unit ratio of 5%.

The entrance and exit of the beam are cut at a Brewster angle and polished and polished precisely. Yb: KYW is a Brewster angle of 63.7 ° defined by θ = tan ⁻¹ (n) in air. Here, n represents a refractive index.

  3A is a block diagram of a diode-pumped solid-state laser device according to an embodiment of the present invention as viewed from a direction perpendicular to an incident plane, and FIG. 3-B is a diode-pumped solid-state laser device according to an embodiment of the present invention. It is the block diagram which looked at the solid laser device seen from the incident plane.

  Such asymmetrical diffractive properties can be improved by the optical pumping structure of FIGS.

  When the anisotropic laser crystal 11 is pumped by a vertical pumping structure, the optical pumping structure of the present embodiment includes a laser diode 14 having a narrow stripe pattern, a pump radiation source 14 of a laser diode bar or a laser diode array, An aspheric lens 15 and two first and second cylinder lenses 16 and 17 are used to make this pump beam into parallel light, and a converging lens 18 is used to focus the beam on the entrance surface of the laser crystal 11. .

  At this time, the pump radiation source 14 is smaller in the first direction (early direction) and has a larger asymmetric cross section and brightness in the second direction (slower direction) perpendicular (orthogonal) to the first direction. Generate a beam.

  The aspherical lens 15 makes the pump beam emitted from the pump radiation source 14 parallel in the first direction, and the first cylinder lens 16 serves to enlarge the size of the pump beam that has passed through the aspherical lens 15. The second cylinder lens 17 located next to the one cylinder lens 16 (on the right side) serves to make the pump beam parallel in the second direction.

In order to reflect the laser beam oscillated by the first and second laser resonator mirrors 19a and 19b to be focused on the laser crystal 11 and to pass the pump beam output from the laser diode 14, the first and second The dichroic resonator mirrors 20a and 20b are arranged. With a diode-pumped solid state laser, such an optical pumping structure can be arranged at one or both ends of an anisotropic laser crystal.

  When the slow axis of the laser diode 14 is oriented in the direction perpendicular (orthogonal) to the incident plane, the initial polarization of the laser diode 14 is in a plane perpendicular to the incident plane of the anisotropic laser crystal 11 (orthogonal plane). This will be the TE polarization mode.

  On the other hand, although there is a special incident angle called Brewster angle (θ), in the TM (Transverse Magnetic) polarization mode, reflection at the boundary becomes zero. That is, in order to minimize pumping power loss and increase pumping efficiency, the polarization direction must be converted from TE mode to TM mode.

  Therefore, in order to rotate the polarization direction (polarization direction) by 90 °, the half-wave plate as the polarization rotation device 21 is arranged in the optical pumping structure. The half-wave plate (21) is arranged at a position where the pump beam becomes parallel light.

  In order to obtain a high pump beam density, it has been proved that the beam characteristics of the laser diode 14 may be improved by using the aspherical lens 15 to make the beams of the fast axis and the slow axis in parallel.

  Due to the difference in divergence angle, the beam output from the laser diode 14 has a larger beam size in the fast axis direction than the beam size in the slow axis direction at the position of the aspheric lens 15.

  In order to correct such asymmetry, it is necessary to increase the beam size only in the slow axis direction.

  That is, in order to increase the beam size in the slow axis direction, the cylinder lenses 16 and 17 are arranged in an optical pumping structure. The first cylinder lens 16 serves to enlarge the size of the pump beam in the second direction, and the second cylinder lens 17 serves to make the pump beam parallel in the second direction.

Here, the ratio of the magnifying glass corresponds to the ratio of the beam quality factor M ² of the laser diode 14 to the fast axis direction and the slow axis direction.

  In order to obtain a beam converged inside the anisotropic laser crystal 11 in a substantially circular spot form, the beam size in the early axial direction is slow at the position of the converging lens 18 in consideration of the refractive index of the anisotropic laser crystal 11. A little smaller than the axial beam size.

  FIG. 4-a is a block diagram of a diode-pumped solid-state laser device according to another embodiment of the present invention viewed from a direction perpendicular to the incident plane, and FIG. 4-b is a diagram of another embodiment of the present invention. It is the block diagram which looked at the diode pumped solid-state laser apparatus from the incident plane.

  As an alternative to the cylinder lenses 16 and 17 employed in the optical pumping structure, as shown in FIGS. 4A and 4B, first, an aspheric lens 25 is used to parallelize the laser diode beam in the fast axis direction. After making, it can be made parallel in the slow direction using the cylinder lens 27.

  Next, the collimated beam having an elliptical cross section that is enlarged along the slow axis is converged to the anisotropic laser crystal 11 by the converging lens 18 to obtain a substantially circular spot.

2 is a cross-sectional view and an axial direction of a narrow striped asymmetric beam generated by a laser diode. It is a drawing showing the axis of the laser medium and the direction of the Brewster angle in the incident plane. It is the block diagram which looked at the diode pumped solid-state laser apparatus by one embodiment of this invention from the orthogonal | vertical direction with respect to the incident plane. It is the block diagram which looked at the diode pumped solid-state laser apparatus by one embodiment of this invention from the incident plane. FIG. 6 is a configuration diagram of a diode-pumped solid-state laser device according to another embodiment of the present invention viewed from a direction perpendicular to an incident plane. FIG. 6 is a configuration diagram of a diode-pumped solid-state laser device according to another embodiment of the present invention as seen from an incident plane. It is a figure which shows the spectrum of absorption (Absorption: solidline) and emission (Emission: dottedline) by the polarization | polarized-light of anisotropic laser crystal Yb: KYW at room temperature. It is a figure which shows the image of anisotropic laser crystal Yb: KYW by one embodiment of this invention with a photograph.

DESCRIPTION OF SYMBOLS 10 ... Narrow striped pattern 11 ... Anisotropic laser crystal 12 ... Polarization direction 13 ... A axis 14 ... Laser diode 15, 25 ... Aspherical lens 16 ... First cylinder lens 17, 25 ... Second cylinder lens 18 ... Converging lens 19a ... first laser resonator mirror, 19b ... second laser resonator mirror ,
20a ... first dichroic resonator mirror, 20b ... second dichroic resonator mirror 21 ... polarization rotation device H ... height, W ... width, θ ... Brewster angle

Claims (7)

Along the optical axis,
A pump radiation source for generating a pump beam having a smaller asymmetric cross section and brightness in a second direction (slow direction) that is smaller in a first direction (early direction) and perpendicular to the first direction; An aspheric lens that collimates the pump beam output from the first direction;
A polarization rotation device that rotates the polarization direction of the pump beam that has passed through the aspheric lens by 90 ° in order to change the polarization direction from the TE mode to the TM mode;
A first cylinder lens that enlarges the size of the pump beam that has passed through the polarization rotator in the second direction;
A second cylinder lens that collimates the pump beam that has passed through the first cylinder lens in the second direction;
A converging lens;
By reconstructing the intensity distribution in the direction of the horizontal axis inside the anisotropic laser crystal, the finally focused beam has an almost circular spot with an absorption length, so that it operates in the TEM ₀₀ mode. Formed by a tungstate medium doped with Yb ³⁺ ions, having characteristics and laser characteristics, having first and second axes crystallographically orthogonal to each other, these axes being the polarization of the pump beam And anisotropic laser crystals having different absorptions depending on each other, and
Making the asymmetric pump beam output from the pump radiation source into parallel light by the aspheric lens, the first cylinder lens, and the second cylinder lens;
The converging lens converges a pump beam collimated by the aspheric lens, the first cylinder lens, and the second cylinder lens on a surface of a beam entrance of the anisotropic laser crystal,
A first dichroic resonator mirror is disposed between the converging lens and a beam entrance of the anisotropic laser crystal;
A second dichroic resonator mirror is disposed at the beam exit of the anisotropic laser crystal, laser light is output to the first dichroic resonator mirror, and the reflected light is output to the converging lens. A first laser resonator mirror that is incident on the beam entrance of the anisotropic laser crystal together with a pump beam from
It said second outputs laser light to dichroic resonator mirror, is incident on the reflected light beam outlet of the anisotropic laser crystal, and a second laser resonator mirror is arranged,
Pumping the anisotropic laser crystal using a pump beam output from the pump radiation source;
Laser device. The direction of polarization of the pump beam output from the pump radiation source is placed along the a-axis of the anisotropic laser crystal so that the anisotropic laser crystal can absorb the pump beam to the maximum extent. ,
The laser device according to claim 1. The anisotropic laser crystal is cut at a Brewster angle at a beam entrance and a beam exit, and the Brewster angle increases according to a ratio of a refractive index of the laser crystal to an air refractive index.
The laser device according to claim 1. The anisotropic laser crystal is a Yb: KYW or Yb: KGW crystal, which is a tungstate medium to which the Yb ³⁺ ions are added.
The laser device according to claim 3. The polarization rotation device is a half-wave plate;
The laser device according to claim 1. The pump radiation source is any one selected from a laser diode, a laser diode bar, and a laser diode array in which each emitter emitting a pump beam has a linear shape.
The laser device according to claim 1. The anisotropic laser crystal is cut at a Brewster angle,
The laser device according to claim 1.