JPH03166529A - Second harmonic generator - Google Patents

Abstract

PURPOSE:To generate coherent light of a short wavelength collimated to high flatness by providing a single or plural pieces of fibers or waveguides and a collimator to condense the second harmonic Cerenkov beam from the fibers or the waveguides and forming the collimator of a diffraction grating. CONSTITUTION:The light beam 1 from a stimulating light source, such as semi conductor laser, is condensed by a lens and is made incident to the waveguide 3 for second harmonic generation having a nonlinear optical characteristic. The stimulating light propagating in the waveguide 3 radiates the Cerenkov type second harmonic wave 4 as a circular conical beam toward a substrate 2. The radiated beam 4 is collimated by the diffraction grating 5 inscribed on the end face of the substrate and progresses in the form of collated plane beams 6. The collimated plane waves having high flatness are generated in this way.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a second harmonic generation device using Cerenkov radiation, which is useful as a coherent short wavelength light source for optical recording of optical discs, laser printers, color printers, etc. Regarding. [Conventional technology] Conventionally, Nonlinear Optical Materials, Extended Abstracts (1985) Page 9
7 (Nonlinear Optical Mat
ericals-Extendad abstract
, 1985, p., 97), nonlinear optical organic materials are made into a fiber-like device, coherent first-order light is input to this device, and the second harmonic of Cerenkov radiation is obtained. Technologies for generating waves are known. However, this radiation beam had a cone-like shape, and as it was, it could not be focused on a high-quality spot, resulting in a crossroad. In addition, regarding the generation of second harmonics by Cerenkov radiation from a waveguide using lithium diobate (LiNbOa) as a substrate, CLEO'87, Technical Digest, page 198 (CLEO'87, Techni
cal Digest p p. 198).

This method can generate secondary light with a wavelength half that of the l-order light with high conversion efficiency, and it is relatively easy to phase match these two light waves with different wavelengths. It has many advantages, such as: However, as shown in Figure 2, this method uses a radiation mode from a narrow linear waveguide into the substrate, so it becomes a diverging light in the radiation direction, and a collimated parallel light in the perpendicular direction. It is light, and the beam wavefront is distorted. For this reason, it is not possible to condense the light and focus it on a single point, which is a major drawback that hinders its application to optical discs, laser printers, and the like.

[Problem to be solved by the invention]

All of the above conventional techniques have a problem in that insufficient consideration has been given to the method of focusing the beam. In other words, even if we look at the conventional example shown in Figure 2, there is no innovation in the shape of the waveguide substrate; it is just a block made of flat surfaces, and the beam that passes through this substrate has large aberrations. However, in the conventional examples so far, no means for correcting this aberration has been disclosed. The present invention has been made for the purpose of providing a second harmonic generation device that is compact, capable of direct light intensity modulation, and capable of generating short-wavelength coherent light that is collimated to a high degree of flatness. [Means for Solving the Problems] The present invention adds an aberration correction function or an intensity distribution correction function to the waveguide substrate or the cladding part of the fiber.
It increases the flatness of the wavefront created by the beam emitted into the air, and adds an optical system that narrows it down to a single diffraction-limited spot when focused with a lens.

That is, more specifically, a conical Cerenkov beam emitted from a nonlinear optical medium in the form of a fiber or a waveguide is processed by a concentric diffraction grating centered on the core of the fiber or the waveguide. It is designed to diffract each light beam and create parallel plane waves after passing through it. On the other hand, in the process of manufacturing the diffraction grating, production errors are unavoidable, and it is impossible to perfectly align the center of the concentric diffraction grating with the position of the fiber or waveguide. Therefore, the present invention proposes the introduction of a sine condition as a characteristic that a microscope objective lens should have, that is, a condition for eliminating coma aberration in order to ensure a constant angle of view. That is, the end face of the fiber cladding or the end face of the waveguide substrate is made into a spherical surface, and the concentric diffraction grating is carved on the spherical surface. Here, the center of the spherical surface is made to coincide with the center of the input end face of the fiber core or waveguide.

[Effect]

As above. By using the concentric diffraction grating according to the present invention, a conical Cherekov beam is collimated as a parallel plane wave, which in turn makes it possible to narrow it down to a single point with high diffraction-limited precision using a lens.

Furthermore, by carving the diffraction grating on a spherical surface, even if there is some deviation between the center of the concentric diffraction grating and the position of the fiber core or waveguide, aberrations will not become large. Even in such cases, lenses can still be used to focus on a single point with high diffraction-limited precision. [Example] Hereinafter, a first example of the present invention will be described with reference to FIG.

A light beam 1 from an excitation light source such as a semiconductor laser is focused by a lens and made to enter a second harmonic generation waveguide 3 having nonlinear optical characteristics. The excitation light propagating through the waveguide 3 emits Cerenkov type second harmonic waves 4 as a conical beam toward the substrate 2. Here, the substrate 2 is made of, for example, a nonlinear optical crystal such as lithium niobate.

The radiation beam 4 is collimated by a diffraction grating 5 cut into the edge of the substrate and travels as a parallel plane wave 6.

Since the beam thus collimated has a flat wavefront, it is focused by the lens 7 as a spot narrowed down to a single point by the diffraction limit.

FIG. 3 shows a second embodiment. That is, a beam 1 from a semiconductor laser is made incident on a fiber-shaped nonlinear optical waveguide 3 to generate a Cerenkov beam 4. A concentric diffraction grating 5 is carved on the end face of the fiber 8, and the cone-shaped Cerenkov beam is diffracted into parallel light 9, which can be focused by a lens.

FIG. 4 shows a third embodiment of the invention. In other words, the exit end surface O of the Cerenkov beam is changed to the entrance end surface 7 of the waveguide.
is processed into a spherical surface centered on , and a diffraction grating 5 is placed on top of it.
shall be engraved. In this way, even if the center of the diffraction grating and the waveguide are slightly shifted or tilted, the collimated beam 6 can be made into parallel light that maintains good flatness. FIG. 5 shows a fourth embodiment of the present invention. In other words, by making the output end face 10 a spherical surface centered on the center 7 of the waveguide, the sine condition is satisfied, and even if the center of the fiber and the center of the concentric circle of the diffraction grating are shifted or tilted, the flatness is maintained. A parallel beam 6 with a high angle can be obtained.

The concentric diffraction gratings described above can be shaped by, for example, mechanical scoring using a ruling engine or an etching process using an electron beam.

〔Effect of the invention〕

As described above, according to the present invention, a collimated plane wave with high flatness can be obtained even if Cerenkov radiation, which is easy to phase match when generating the second harmonic, is emitted in a cone shape. It becomes possible to focus this on a diffraction-limited spot using a lens, making it possible to apply it to common information processing equipment. This makes it possible, for example, to increase the recording density of an optical disk by four times or more, or to increase the printing resolution of a laser beam printer to twice that of the conventional technology.

[Brief explanation of the drawing]

FIG. 1 is a schematic perspective view of a second harmonic generator according to an embodiment of the present invention, FIG. 2 is a perspective view of a conventional device, and FIGS. 3 to 5 are other embodiments of the present invention. FIG. 2 is a schematic perspective view of a second harmonic generator. DESCRIPTION OF SYMBOLS 1... Semiconductor laser beam, 2... Nonlinear optical crystal, 3... Waveguide, 4... Cerenkov beam, 5...
...Diffraction grating, 6...Parallel beam, 7...Lens,
8...Fiber cladding, 9...Parallel beam, 1o
...Spherical surface. Part Z Part 1: 3rd and 5th time

Claims (1)

[Claims] 1. Comprising a single or multiple fibers or waveguides made of a nonlinear optical material, and a collimator for condensing a second harmonic Thierenkov beam from the fibers or waveguides. , a second harmonic generation device characterized in that the collimator is a diffraction grating. 2. A second harmonic generation device characterized in that the diffraction grating is carved on the end face of a fiber cladding or the end face of a waveguide substrate. 3. A second harmonic generation device characterized in that the end face of the fiber cladding or the end face of the waveguide substrate is a spherical surface, and the diffraction grating is carved on the spherical surface.