JPH05297253A - Coupled lens device - Google Patents

Abstract

PURPOSE:To provide a coupling lens device which easures easy acquisition of components and adjustment, does not require correction of the anisotropy in the radiation angle distribution, allows constituting a compacted optical system, and enables high-efficiency coupling with each optical fiber having a small numerical operture, by installing a cylindrical lens apart from the end faces of laser diodes so that the axis of the cylinder is identically directed to the longitudinal axis of the array of laser diodes. CONSTITUTION:A cylindrical lens 12 is installed in proximity to an array 11 of laser diodes of the gain waveguide type which is a semiconductor laser element as a laser beam source for energization having anisotropic radiation angle, and the energizing laser beam from the array 11 is led to an optical fiber array 13 which is arranged behind the cylindrical lens 12. This lens 12 is made of a material of homogeneous or refractive index distribution type such as molten quartz or optical glass, preferably having a dia. of 100-700mum, typically being 400mum, and having a focal distance of 100-500mum, typically 300mum. The laser diode may have a length of opening (light emission width) of 100mum approximately and have anisotropic radiation angle being 10deg.X30deg. approximately.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coupling lens device, and more particularly to a coupling lens device for coupling a laser diode to an optical fiber or the like for sending pump laser light to a laser diode pump fixed laser device.

[0002]

2. Description of the Related Art In order to increase the output of a laser diode pumped solid-state laser device, pumped laser light from a laser diode is coupled to an optical fiber, and a plurality of the optical fibers are bound to form a high-brightness pumping light source, A method of forming an image of this in a solid-state laser medium (laser rod) is described in, for example, a document by the present inventors, S. Kubota, M. Oka, Y. Kaneda a.
nd H. Masuda, "Thermal aberration analysis of a laser
-diode-pumped Nd: YAGlaser ", SPIE Vol.1354 Internat
ional Lens Design Conference (1990), pp.572-580
Are disclosed in.

The laser diode used in this case is, for example, a gain-guided laser diode which serves as a line light source having an aperture length (light emission width) of about 100 μm, and its radiation angle distribution is, for example, a full angle at half maximum of 10 ° × 30 °. Has anisotropy. Therefore, it is necessary to correct the anisotropy by using a so-called Brewster-Anamorphic telescope or the like, and to select an optical fiber having a core diameter and a numerical aperture suitable for the horizontal aperture length and the radiation angle of 10 °.

[0004]

By the way, according to the Lagrange-Helmholtz invariant law of geometrical optics, when the aperture length and emission angle of the light source are determined, in order to couple without loss, the fiber core diameter and numerical aperture are The product of must always be constant. Therefore, in order to make a higher brightness light source, it is desired to couple with a fiber having a small core diameter, but at this time, the numerical aperture of the optical fiber must be increased. When the numerical aperture of the optical fiber becomes large, similarly, the numerical aperture of the light flux imaged in the solid-state laser rod also becomes large, the overlap with the modes of the solid-state laser becomes poor, and the efficiency is lowered.

The present invention has been made in view of the above circumstances, and when coupling laser light from a laser diode having anisotropy in radiation angle to an optical fiber, readily available parts can be used. , Easy to adjust, no need to correct anisotropy of radiation angle distribution, compact optical system can be configured, and a coupling lens device that can couple with high efficiency to an optical fiber with a small numerical aperture is provided. It is intended.

[0006]

SUMMARY OF THE INVENTION A coupling lens device according to the present invention is a coupling lens device for coupling laser light from a laser diode array having anisotropy of radiation angle to an optical fiber array. The problem described above is solved by using a cylindrical lens having a shape, the longitudinal direction of which is arranged in the horizontal direction of the laser diode array, and which is arranged away from the end face of the laser diode.

The cylindrical lens is made of fused quartz or optical glass, and is made of a homogeneous or gradient index material.
It is preferred to have a diameter of ˜700 μm, typically 400 μm. The focal length of this cylindrical lens is 100μ
m to 500 μm, typically about 300 μm. As the laser diode, a laser diode having an opening length (light emission width) of about 100 μm and anisotropy of radiation angle of about 10 ° × 30 ° can be used. The cylindrical lens may be used at a distance of about 100 μm from the end face of the laser diode, and may be coupled with high efficiency to an optical fiber having a numerical aperture of about 0.1.

It is considered that each fiber of the optical fiber array is bundled to form a high-brightness pumping light source, and this high-brightness pumping light source is used for introducing pumping light into the solid-state laser device. This solid-state laser device is, for example, a laser resonator formed by disposing a laser medium such as Nd: YAG and a nonlinear optical crystal element such as KTP (KTiOP ₄ ) or a wavelength conversion element between a pair of reflecting mirrors. Can be used. As the laser medium in the laser resonator of this solid-state laser device, in addition to Nd: YAG, Nd: YVO ₄ , Nd: B
EL, LNP, or the like is used, and as the wavelength conversion element or the nonlinear optical crystal element, in addition to the KTP, L
N, BBO, LBO, etc. are used. Further, as the laser resonator, for example, a fundamental wave laser light generated in a laser medium excited by excitation light is resonated so as to pass through a nonlinear optical crystal element provided inside the resonator, A quarter-wavelength for generating the second harmonic laser light of II and for suppressing the coupling due to the sum frequency generation between the two polarization modes of the fundamental laser light to perform stable oscillation in two modes. A birefringent element such as a plate provided in the resonator path can be used.

[0009]

The cylindrical lens can use, for example, a core portion of an optical fiber, is easily available, can be easily adjusted, and it is not necessary to correct the anisotropy of the radiation angle distribution of the laser diode. A compact optical system can be constructed, and a high coupling efficiency close to 90% can be obtained for an optical fiber having a small numerical aperture.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram showing the schematic structure of an embodiment of a combined lens device according to the present invention. In FIG. 1, a cylindrical lens 12 is arranged in the vicinity of a gain-guided laser diode array 11 which is a semiconductor laser element as a laser light source for excitation having anisotropy in emission angle, and the cylindrical lens 12 is arranged behind the cylindrical lens 12. The excitation laser light from the laser diode array 11 is guided to the optical fiber array 13 arranged at. The laser diode array 11 is
In the active layer 11A as shown in FIG. 2, a plurality of light emitting portions (openings) 11E shown in FIG. 1 are arranged with a light emission width w and an arrangement pitch p. As specific numerical values, w is about 100 μm and p is about 300 μm.

The cylindrical lenses 12 are arranged so that the lengthwise direction of the cylindrical lenses 12 having a diameter D (for example, about 400 μm) is aligned along the arrangement direction (horizontal direction, junction parallel direction) of the laser diode array 11. ing. The emission angle (divergence angle) α of the emitted light in the horizontal plane of the laser diode array 11 is smaller than the emission angle (divergence angle) β of the emitted light in the vertical plane shown in FIG. Specifically, for example, α
Is about 10 ° and β is about 30 °. The laser light emitted from the laser diode array 12 is substantially collimated (collimated light) in the vertical plane by the cylindrical lens 12 and guided to each optical fiber 13F of the optical fiber array 13. Laser diode array 1
Although the luminous flux of the outgoing light of No. 2 in the horizontal plane is not collimated, since the divergence angle α is small and the diameter of the cylindrical lens 12 is sufficiently small, the optical fiber array 13 is sufficiently efficient.
Is coupled to each optical fiber 13F.

Each optical fiber 1 of the optical fiber array 13
3F is bundled into a cylindrical shape to form an optical fiber bundle 13B as shown in FIG. 3, and high-intensity pump laser light is emitted from the end portion. The high-intensity excitation laser light is condensed by the lens 22 and is incident on the laser medium 26 using, for example, Nd: YAG via the concave mirror 24 of the laser resonator 23 and the quarter-wave plate 25. The concave surface inside the concave mirror 24 of the resonator 23 is the reflecting surface 14R. The laser medium 26 such as Nd: YAG generates the fundamental laser light LA (ω) in response to the incident excitation light, and the fundamental laser light LA (ω) is generated.
Passes through the nonlinear optical crystal element (wavelength conversion element) 27 using, for example, KTP (KTiOPO ₄ ), reaches the plane mirror 28, and is reflected by the reflection surface 28R.

The nonlinear optical crystal element 27 such as KTP is provided with the above-mentioned fundamental wave laser light LA by type II phase matching.
The second harmonic laser light LA (2
ω) is generated. For example, if the wavelength λ of the fundamental wave laser light LA (ω) is 1064 nm, the second harmonic laser light LA
The wavelength of (2ω) is 532 nm which is λ / 2. Concave mirror 2
The reflection surface 24R of No. 4 has the excitation light (for example, the wavelength 808n
m) has a high transmittance and a characteristic such as a high reflectance with respect to the fundamental wave laser light LA (ω) generated in the laser medium 26, and the reflecting surface 28R of the plane mirror 28 has a fundamental wave laser It has such characteristics that it has a high reflectance for the light LA (ω) and a high transmittance for the second harmonic laser light LA (2ω). These reflecting surfaces 24R and 28R can be formed by so-called dichroic mirrors. Therefore,
Fundamental laser light LA (ω) generated in the laser medium 26
Travels back and forth between the reflecting surface 24R and the reflecting surface 28R of the laser resonator 23, and the laser light is oscillated. In addition,
The quarter-wave plate 25 is set in an orientation inclined by an azimuth angle θ = 45 ° with respect to the nonlinear optical crystal element 27, and is for stabilizing the power of laser light in each part of the resonator 23. Is.

That is, the quarter wave plate 25 is
The applicant of the present application has previously disclosed Japanese Patent Application Laid-Open No. 1-220879 and
Description and Drawing of Japanese Patent Application No. 2-125854, Japanese Patent Application No. 3
-17068 is a birefringent element used based on the technique disclosed in the specification, drawings, etc. of No. 17068, and is a quarter wavelength set in an azimuth inclined by an azimuth angle θ = 45 ° with respect to the nonlinear optical crystal element 27. By passing through the plate 25, the power of the laser light in each part of the resonator 23 is stabilized.

That is, by inserting the quarter-wave plate 25, (i) the non-linear coupling between the polarization modes due to the sum frequency generation is eliminated, and the mode competition between the polarization modes can be prevented. (Ii) Since the spatial phase difference between the two polarization modes is 90 °, the spatial hole burning effect can be suppressed by oscillating the two polarization modes, and stable oscillation of two vertical modes (two polarization modes) can be obtained. .. That is, the effect is obtained.

In such a configuration, each pump laser beam from the plurality of light emitting portions 11E of the laser diode array 11 passes through the cylindrical lens 12 and the optical fiber array 1
It is possible to increase the output of the solid-state laser device by being coupled to the respective optical fibers 13F of No. 3 and bundling the plurality of optical fibers 13F to form a high-brightness excitation light source by the optical fiber 13B.

By the way, the product of the diameter and the numerical aperture of a fiber bundle composed of a plurality of optical fibers 13F is the product of the spot size of the pumping light in the laser medium (Nd: YAG rod) 26 and the numerical aperture of the convergent light. equal. Therefore, when the diameter of the fiber bundle is increased with the magnification of the focusing system kept constant, the spot size in the laser medium 26 also increases. In order to maintain the overlap with the laser mode, the resonator length (effective length) of the laser resonator 23 must be increased. Similarly, when the numerical aperture of the fiber bundle increases, the numerical aperture of the converged light also increases, and the overlap with the laser mode deteriorates. On the other hand, if the numerical aperture of the converged light is reduced by increasing the magnification of the condensing system, the spot size increases, so that the resonator length must be lengthened. Therefore, in order to increase the diameter of the fiber bundle in order to increase the output while keeping the laser cavity length constant, it is necessary to reduce the numerical aperture of the fiber bundle.

Here, in the embodiment of the present invention, a cylindrical lens 12 having a diameter of about 400 μm is arranged between the laser diode array 11 and the optical fiber array 13. Although an actual design example will be described below, for the sake of convenience of ray tracing, the parallel light flux emitted from the optical fiber 16F side is correctly aligned on the line of the laser diode end face by the cylindrical lens 12, contrary to the actual emission direction of the laser light. Evaluate whether it is imaged. Since the vertical emission angle of the laser diode is 30 °, the numerical aperture of the cylindrical lens 12 must be 0.4 or more.

Assuming that the material of the cylindrical lens 12 is homogeneous fused silica, the refractive index at a wavelength of 810 nm is 1.4.
Since it is 53, the focal length is 320 μm when the diameter D is 400 μm. Here, in the case of a luminous flux diameter of 260 μm corresponding to a numerical aperture of 0.4, as shown in FIG.
The spherical aberration (vertical axis) is 10 μm or less. The spherical aberration of 10 μm is 10 μm / 320 μm on the optical fiber.
Since it corresponds to the spread of the luminous flux of 0.03, the numerical aperture is 0.
It is well coupled to one optical fiber. Off-axis aberration of 20 μm
A coma of the same degree is similarly sufficiently coupled to an optical fiber having a numerical aperture of 0.1.

When the numerical aperture is 0.5 or more, the gradient index optical fiber having a refractive index distribution in the radial direction is used as the cylindrical lens 12.
Can be used to reduce the occurrence of aberration. According to the cylindrical lens 12 in which an optical fiber having a diameter of 400 μm and a refractive index of 1.506 is provided with a refractive index distribution such that the refractive index increases toward the outside by ion exchange or the like, the spherical aberration is 10 μm as shown in FIG. Degree, off-axis top 2
It becomes about 0 μm. Refractive index distribution, the radial position of the cylindrical lens refractive index at (radial coordinate) r n (r) n ( r) = 1.506 × (1+ (ar) 2/2), a = 0. It is given as 063 / mm.

At this time, the focal length of the cylindrical lens 12 is 3
At 80 μm, the beam diameter on the optical fiber is 380 μm in the paraxial approximation calculation.
m. Generally, the aberration of a cylindrical lens is relatively large, but the focal length itself is small with a small lens, so
There is an advantage that the absolute value of the aberration becomes small.

Further, since the cylindrical lens 12 has no horizontal refracting power, it is directly coupled to the optical fiber at an emission angle of 10 ° of the laser diode. Considering the numerical aperture in the horizontal direction, sin 10 ° = 0.17, which is small. Therefore, according to the approximate calculation, even if the optical fiber is directly coupled to an optical fiber having a numerical aperture of 0.1, the coupling is about 90%. Efficiency is obtained. If the vertical radiation angle is 30 °, sin 30 °
= 0.5 or more is assumed.

In this way, the laser light from the laser diode array 11 having the anisotropy of the emission angle distribution with a full angle at half maximum of, for example, about 10 ° × 30 ° is used by using the core of the optical fiber as the cylindrical lens 12. It is introduced into the optical fiber array 13 through the cylindrical lens 12, and the optical fiber
A high-intensity excitation light source can be made by bundling the optical fibers 13F of the array 13. In this case, the readily available core portion of the optical fiber can be used as a coupling cylindrical lens, and adjustment is easy because it does not depend on the position even if it moves in the lengthwise direction of the cylindrical lens. It is not necessary to correct the anisotropy of the radiation angle distribution, an extremely compact optical system can be constructed, and the numerical aperture is small (for example, 0.
The effect is that a high coupling efficiency of about 90% can be obtained for an optical fiber.

The present invention is not limited to the above-mentioned embodiments. For example, the aperture length (emission width) of the laser diode, the full width at half maximum of the radiation angle distribution, the material of the cylindrical lens, the diameter, the focal length, and the laser. Needless to say, the distance between the end face of the diode and the cylindrical lens, the numerical aperture of the optical fiber, and the like are not limited to the specific examples described above.

[0025]

As is apparent from the above description, according to the coupling lens device of the present invention, the cylindrical lens is arranged such that the length direction of the cylinder is the length direction of the laser diode array. By arranging the laser diode away from the end face, it is not necessary to correct the anisotropy of the radiation angle distribution of the laser diode, and a compact optical system can be used, with a high coupling efficiency close to 90% for an optical fiber with a small numerical aperture. Arrays can be combined and a high intensity pump source can be realized. For the cylindrical lens, for example, a core portion of an optical fiber or the like can be used, it is easily available, and adjustment is easy. As this cylindrical lens, a homogeneous or gradient index material of fused quartz or optical glass is used, and is 100 μm to 700 μm, typically 4 μm.
Those having a diameter of 00 μm can be used.

Excitation laser light from a high-intensity excitation light source produced by combining a laser diode array with an optical fiber array by such a coupling lens is introduced into a solid-state laser device, and the solid-state laser device is excited. Contributes to higher output.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of a basic embodiment of a coupling lens device according to the present invention and an application example thereof.

FIG. 2 is a diagram for explaining coupling of laser light from the laser diode of the embodiment to an optical fiber.

FIG. 3 is an end view of a bundle of optical fibers used in the embodiment.

FIG. 4 is a diagram showing an example of spherical aberration of a cylindrical lens used in the example.

FIG. 5 is a diagram showing another example of the spherical aberration of the cylindrical lens used in the example.

[Explanation of symbols]

 11 ... Laser diode array 12 ... Cylindrical lens 13 ... Optical fiber array 13F ... Optical fiber 22 ... Lens 23 ... Laser Resonator 24 ... Concave mirror 25 ... Quarter wave plate 26 ... Laser medium 27 ... Nonlinear optical crystal element 28 ... Plane mirror

Claims (2)

[Claims] 1. A coupling lens device for coupling laser light from a laser diode array having anisotropy of radiation angle to an optical fiber array, wherein the coupling lens device has a cylindrical shape, and the longitudinal direction of the cylinder is the laser. A coupling lens device characterized by using a cylindrical lens which is arranged in a horizontal direction of a diode array and is arranged apart from an end face of a laser diode. 2. The cylindrical lens is made of fused silica or a homogeneous or gradient index material of optical glass, and has a thickness of 100 μm.
2. The coupling lens device according to claim 1, having a diameter of m to 700 μm, typically 400 μm.