JPH04225335A - Light source device - Google Patents

Abstract

PURPOSE:To restrain the useless spread of the luminous flux of a fundamental wave, to reduce stray light and to improve S/N at the time of receiving light by obtaining parallel rays of light for both luminous fluxes B1 and B2 of secondary higher harmonic and the fundamental wave. CONSTITUTION:The wave surface of the luminous flux of the fundamental wave B1 radiated from an optical fiber type secondary harmonic producing element 4 as a spherical wave is deformed by a conversion surface 52 and advances as the parallel rays of light. Therefore, the parallel rays of light are simultaneously obtained for both luminous fluxes of the secondary higher harmonic and the fundamental wave B1 and B2.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is directed to generating a second harmonic from a laser beam using a second harmonic generating element, and simultaneously extracting the second harmonic and the original fundamental wave as parallel light beams. This article relates to a light source device that can be used.

0002

2. Description of the Related Art A nonlinear optical effect is a phenomenon in which when light enters a medium, polarization occurs in proportion to a higher-order term equal to or higher than the square of the electric field of the light, and this phenomenon generates second-order harmonics. Materials containing the above medium are called nonlinear optical materials,
Inorganic materials such as KH2PO4 and LiNbO3 have been put into practical use. Furthermore, recently, organic materials represented by 2-methyl-4-nitrileaniline (MNA) have also attracted attention because they have large nonlinear optical constants.

[0003] A wavelength conversion element that uses the above-mentioned nonlinear optical material as a second-order harmonic generation element to halve the wavelength of a low-power laser beam such as a semiconductor laser has been put into practical use. This wavelength conversion element is designed to confine the fundamental wave (semiconductor laser light) for harmonics at high energy density and to lengthen the interaction length with the harmonics.

[0004] Therefore, as the form of the second harmonic generating element, for example, an optical waveguide type is used. In this method, a long and narrow optical waveguide is formed on a substrate to confine and propagate light, and an overlayer is coated on top of it. However, in order to extract the second harmonic generated in the optical waveguide, etc. , the optical waveguide must have a structure that can accommodate the propagation phase velocity of the second harmonic of the wavelength. That is, the fundamental wave and the second harmonic must be phase matched. Various methods can be considered to achieve this phase matching, but the simplest method is to use the Cerenkov radiation method.

In this method, as shown in FIG. 18, a second harmonic is generated from the light propagating through the optical waveguide 11 at point A and leaks into the substrate 12 and overlayer 13 at an angle θ. Then, if the equal phase front of the second harmonic that is emitted again in the θ direction after a unit time and the equal phase front of the second harmonic match, this angle θ
The second harmonic is emitted in the direction. If the refractive index of the substrate for the fundamental wave is ns (ω), the refractive index of the waveguide is nG (ω), and the refractive index of the substrate for the second harmonic is nS (2ω), then nS (2ω)>nG (ω) )>nS (ω), phase matching is automatically achieved and Cerenkov radiation becomes possible, and therefore this Cerenkov radiation is adopted as the method that can most easily achieve phase matching.

However, in the optical waveguide type second harmonic generation element, since the second harmonic is emitted from the narrow optical waveguide to the substrate, the characteristics of the light beam are crescent-shaped in cross section, which is not good in terms of focusing. I can't say that. Therefore, this second harmonic has the disadvantage that it cannot be focused on a small spot. Therefore, it has been difficult to use second harmonics for writing and reading from, for example, optical storage media having minute pits, such as optical disks.

On the other hand, since the optical fiber type second harmonic generation element is axially symmetrical, the second harmonic spreads in a ring shape and can also be made into parallel light beams. Therefore, a laser light source, an optical fiber type second harmonic generating element that generates a second harmonic from the laser light emitted from the laser light source, and a second harmonic generating element that has a circularly symmetrical inclined surface and that generates a second harmonic from the second harmonic generating element. A light source device equipped with a collimating lens that converts harmonics into parallel light beams has been disclosed (see Japanese Patent Laid-Open No. 1-287531).

According to the light source device having the above configuration, when the laser light emitted from the laser light source is guided to the optical fiber type second harmonic generation element and the second harmonic is generated, the second harmonic is axially symmetrical and conical. The wave spreads out from the end face of the optical fiber as a wave with an equal phase front. FIG. 19 shows this situation, and the second harmonic is generated by the cladding 4 of the optical fiber 4.
2, it becomes a conical beam B and spreads out. Therefore, by passing this second harmonic through a collimating lens that at least partially has a circularly symmetrical inclined surface,
Parallel rays of harmonics can be obtained.

[0009] The collimating lens does not have to have an actual conical shape, but may be one that converts a conical wavefront having an equiphase front into a parallel wavefront. For example, a grating axicon lens to which a diffraction grating is applied may be used (see Japanese Patent Laid-Open No. 2-153328).

0010

[Problem to be Solved by the Invention] However, among the light emitted from the above-mentioned light source device, the second harmonic is converted into a parallel beam, but the fundamental wave is spread out and becomes stray light, which interferes with measurement. . Figure 20 shows this situation,
Even if the second harmonic wave B2 is converted into a parallel light beam, the fundamental wave light beam B1 may or may not pass through the conical lens 5 and spread out.

[0011] For this reason, for example, in an optical disc playback device, even if an attempt is made to utilize the focused second harmonic, if the photodetector is sensitive to the fundamental wave, the fundamental wave that has become stray light will be reflected. The signal enters the photodetector and is added to the signal as a noise component, resulting in poor S/N ratio. Of course, you can cut the fundamental wave by inserting a wavelength filter, but multiple wavelength filters are required to cut the fundamental wave to a sufficient degree. The waves are attenuated, and position control (currently, this type of device uses second harmonics to control the position of the readout point) cannot be performed sufficiently, making it even more difficult to handle.

The present invention provides an optical fiber type second harmonic generation element that generates second harmonic waves based on laser light emitted from a laser light source, and a second harmonic wave emitted from the second harmonic generation element into parallel light beams. It is an object of the present invention to provide a light source device equipped with a collimating lens for conversion, which can prevent unnecessary spread of fundamental wave rays and suppress generation of stray light.

[0013]

[Means for Solving the Problems] A light source device of the present invention for achieving the above object has a collimating lens that converts a fundamental wave emitted from an optical fiber type second harmonic generation element and spread in a spherical shape into parallel light beams. A conversion surface for conversion is formed (claim 1). Further, the collimating lens is formed with a conversion surface that converts the fundamental wave emitted from the second harmonic generation element and spread in a spherical shape into a spherical wave having a different curvature (claim 2).

These "conversion surfaces" may be, for example, spherical surfaces, or surfaces having an annular diffraction grating pattern that performs the same function as a spherical surface. It is preferable that the numerical aperture of the conversion surface is greater than or equal to the numerical aperture of the second harmonic generation element.

[0015]

[Operation] According to the structure of claim 1, the optical fiber type 2
The fundamental wave light flux emitted as a spherical wave from the harmonic generating element has its wavefront deformed by the conversion surface, and travels as a parallel light beam. Therefore, parallel light beams can be obtained simultaneously for both the second harmonic and the fundamental wave, and both can be focused using a lens or the like.

According to the second aspect of the present invention, the fundamental wave light flux emitted as a spherical wave from the optical fiber type second harmonic generation element is emitted as a spherical wave having a different curvature due to the conversion surface. Therefore, it is the same as the case of claim 1 that both the second harmonic and the fundamental wave can be focused using a lens, etc., but in the invention of claim 1, the second harmonic can be focused due to the chromatic aberration of the lens etc. Whereas the spots of the wave and the fundamental wave are necessarily located at different positions, in the invention according to claim 2, it is possible to adjust the imaging position of the spot by inversely utilizing the chromatic aberration of the lens or the like.

[0017]

Embodiments Next, embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a laser light source 1 such as a semiconductor laser, a spherical lens 2 that collimates the laser light generated from the laser light source 1, a condensing spherical lens 3 that collects the collimated light beam, a core 41, and a cladding 42. An optical fiber type second harmonic generation element 4 using a known nonlinear optical material such as MNA for one or both of them, and a light flux of the second harmonic generated from the second harmonic generation element 4 (hereinafter simply "second order harmonic generation element 4"). A conical surface 51 that collimates the
(apex angle 2α) and the fundamental wave light flux (hereinafter simply referred to as “fundamental wave”)
This figure shows a light source device composed of a collimating lens 5 having a spherical surface 52 (radius R) and a refractive index n for collimating the light. Note that the collimator lens 5 is arranged so that its rotational symmetry axis coincides with the rotational symmetry axis (long axis) of the second harmonic generation element 4.

In the above light source device, the second harmonic B2 emitted from the second harmonic generating element 4 forms a constant angle θo with the axis of rotational symmetry, as shown in FIG. Since the collimating lens 5 has a conical surface 51 with an apex angle of 2α, the rotational symmetry axis of the collimating lens 5 is set to the rotational symmetry axis of the optical fiber, and tan α=(n-cos θo)/sin θo. If n and α are chosen as follows, parallel rays can be obtained for the second harmonic. The size of the conical surface 51 is determined by the diameter of the cladding 42, the distance L between the output end of the second harmonic generation element 4 and the lens 5, and the spread angle θo of the second harmonic output from the second harmonic generation element 4. You can decide accordingly. In the embodiment, it is a ring-shaped conical surface with an outer diameter d1 and an inner diameter d2.

Furthermore, if the distance between the output end of the second harmonic generating element 4 and the spherical surface 52 on the optical axis is L, then the center of the spherical surface 52 is set on the axis of rotational symmetry of the optical fiber, and 1/L= (n-
1) If the radius R is set to be /R, a parallel ray can be obtained for the fundamental wave. This condition is the same as the fact that the focal length f and the distance L of the spherical lens are equal.

Furthermore, by making the numerical aperture of the optical fiber equal to the numerical aperture of the spherical lens made of the spherical surface 52, the preferred size of the spherical lens is determined. This condition is as follows. (n1 2 - n2 2 ) 1/2 = D/f However, D is the radius of the spherical surface 52, and n1 is the core 4 for the fundamental wave.
1 and n2 is the refractive index of the cladding 42 for the fundamental wave. If the radius of the spherical lens is smaller than the radius D expressed by this equation, a portion of the fundamental wave will escape depending on the degree, and a certain amount of stray light will occur accordingly.

In the above configuration, the second harmonics are parallel to each other on the conical surface 51, and the fundamental waves are parallel to each other on the spherical surface 52. As a result, a light beam consisting of a fundamental wave is obtained at the center, a light beam consisting of a ring-shaped second harmonic wave is obtained at the outer side, and a light beam consisting of a ring-shaped fundamental wave is obtained at the outer side. The shape of the collimating lens 5 described above is not limited to the shape that is a combination of a spherical surface and a conical surface as shown in FIG. For example, as shown in FIG. 2(a), a grating axicon lens 53 may be used instead of the conical lens. In this case, the output angle θo of the second harmonic, the pitch d of the diffraction grating, and the inclination angle α of the diffraction grating surface have the following relationship: sin θo=λ/θo tan α=(n-cosθo)/sin θo If this is satisfied, parallel rays can be obtained (Figure 2 (
b) Reference). Here, λ is the wavelength of the second harmonic in the air, and n is the refractive index of the material of the diffraction grating.

Furthermore, instead of the spherical lens, an aspherical lens 54 (see FIG. 3), a gradient index lens 55 (see FIG. 4), a grating lens (see FIG. 5) 56, etc. may be used. Further, depending on usage conditions, the parallel light beam consisting of the fundamental wave may be formed only inside or outside the second harmonic (see FIGS. 6 and 7).

Furthermore, the conical surface 51 and the spherical surface 52 may be set outside the second harmonic generating element 4 (see FIG. 8). They may be installed on both sides (see Figures 9-14). Furthermore, the lens that converts the fundamental wave into parallel light may be made of different types of lenses on the inside and outside of the conical lens. For example, as shown in FIG. 15, it may be configured with a spherical lens 52 on the inside and a grating lens 56 on the outside.

In addition, if the collimating lens is coated with a non-reflective coating 60 (FIG. 16), reflections on the surface can be suppressed and stray light can be further reduced. In any of the above cases, the collimating lens 5 can be used to focus the fundamental wave into a parallel beam. Therefore, the rate at which the fundamental wave escapes can be reduced, and the generation of stray light can be suppressed as much as possible.

Furthermore, since the emission angle of the fundamental wave is constant because it is a parallel beam, it is now possible to attenuate the fundamental wave to a sufficient extent by using, for example, a dielectric multilayer reflector, and by using a wavelength filter. The S/N can be improved compared to the case. In addition, when used in an optical disc playback device,
The condensed beam consisting of the second harmonic and the condensed beam consisting of the fundamental wave can be condensed at different positions, with the spot of the second harmonic being used only for reading and the spot of the fundamental wave being exclusively used for position control. By using this, highly accurate reading becomes possible.

Next, an embodiment of the invention according to claim 2 is shown in FIG.
This will be explained using 7. In FIG. 17, parts with the same reference numerals as in FIG. 1 indicate the same members. What is different from FIG. 1 is that the lens 5 is formed with a conversion surface 58 that converts the fundamental wave that is emitted from the second harmonic generation element 4 and spreads in a spherical shape into a spherical wave with a different curvature. The material of the condensing lens 7 is selected so that the second harmonics, which have become parallel light beams, are condensed onto the same spot Q.

With this configuration, it is possible to obtain a spot at the same position Q using the same condensing lens 7, so if a light receiving element is placed at that point, the number of light receiving elements is reduced to 1. This makes it possible to easily perform signal processing in an optical system including a light source device.

[0028]

As described above, according to the light source device of claim 1 of the present invention, the wavefront of the fundamental wave light beam emitted as a spherical wave from the optical fiber type second harmonic generation element is converted by the conversion surface. and travels as parallel rays. therefore,
Since parallel light beams can be obtained for both the second harmonic and the fundamental wave, stray light can be reduced and the S/N ratio at the time of light reception can be improved.

Further, according to the light source device of claim 2, in addition to reducing stray light, the second harmonic and the fundamental wave can be focused at the same point by focusing the light with a lens having an appropriate chromatic aberration. This makes it possible to simplify the configuration of the signal processing system.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an example of a light source device.

FIG. 2 is a diagram showing a modified example in which a part of the collimating lens is configured with a grating axicon lens.

FIG. 3 is a diagram showing a modified example in which a part of the collimating lens is configured with an aspherical lens.

FIG. 4 is a diagram showing a modified example in which a part of the collimating lens is configured with a gradient index lens.

FIG. 5 is a diagram showing a modification example in which a part of the collimating lens is configured with a grating lens.

FIG. 6 is a diagram showing a modified example of the collimating lens in which a parallel light beam consisting of a fundamental wave is formed only inside the second harmonic.

FIG. 7 is a diagram showing a modified example of the collimating lens in which a parallel light beam consisting of a fundamental wave is formed only outside the second harmonic.

FIG. 8 is a diagram showing a modified example of the collimating lens in which the conversion surface is provided outside the second harmonic generation element.

FIG. 9 is a diagram showing a modification example in which the collimating lens is composed of two separate lenses and conversion surfaces are provided on both outer sides.

FIG. 10 is a diagram showing a modification of the collimating lens in which a conversion surface and a conical surface are provided on both sides of the lens.

FIG. 11 is a diagram showing a modified example of a collimating lens in which conversion surfaces are provided on both sides of the lens.

FIG. 12 is a diagram showing a modified example of a collimating lens in which conical surfaces are provided on both sides of the lens.

FIG. 13 is a diagram showing a modification of the collimating lens in which a conversion surface and a conical surface are provided on both sides of the lens.

FIG. 14 is a diagram showing a modification of the collimating lens in which a conversion surface and a conical surface are each provided on one side of the lens.

FIG. 15 is a diagram showing a modified example of a collimating lens in which the conversion surface is composed of lenses of different types.

FIG. 16 is a diagram showing an example in which a non-reflective coating is applied to the lens surface to further reduce stray light.

FIG. 17 is a configuration diagram showing an embodiment of a light source device according to a second aspect of the invention.

FIG. 18 is an explanatory diagram showing the Cerenkov radiation method.

FIG. 19 is a diagram showing a beam emitted in a conical wavefront shape.

FIG. 20 is a diagram showing a fundamental light beam B1 that spreads while passing through a conical lens and not passing through the conical lens.

[Explanation of symbols]

1 Laser light source 4 Second harmonic generation element 41 Core 42 Clad 5 Collimating lens 51 Conical surface 52 Spherical surface

Claims (2)

[Claims] Claim 1: A laser light source, an optical fiber type second harmonic generation element that generates a second harmonic of laser light emitted from the laser light source, and a second harmonic generated from the second harmonic generation element. In a light source device comprising a collimating lens that converts into parallel light beams, the collimating lens is provided with a conversion surface that converts a fundamental wave emitted from the second harmonic generation element and spreading in a spherical shape into parallel light beams. A light source device featuring: Claim 2: A laser light source, an optical fiber type second harmonic generation element that generates a second harmonic of the laser light emitted from the laser light source, and a second harmonic generated from the second harmonic generation element. In a light source device equipped with a collimating lens that converts into parallel light beams, the collimating lens is formed with a conversion surface that converts a fundamental wave emitted from a second harmonic generation element and spreading in a spherical shape into a spherical wave having a different curvature. A light source device characterized by: