Second harmonic generator and information processing system using the same

Abstract: A system for increasing the frequency of a laser beam utilizes focused Cherenkov radiation. The system utilizes re­ flective/refractive properties of continuous optices (8, 5) to achieve increased frequency, which provides a means to form a focussed beam increased density in such application as op­ tical disk technology.

Description: Background of the Invention The present invention relates to an optical system for generating harmonic light which utilizes Cherenkov radiation and is useful as a source of coherent short-wave light. Such light is useful for optical reading or recording, such as in optical disks, laser printers or color printers. Such devices benefit from light having a higher frequency, and correspondingly, a lower wavelength. Shorter wavelength light provides for example, increased resolution in laser printers, and increased data density in optical disks. It has been recognized that notwithstanding the limited optical frequency spectra obtainable from current semiconductor laser devices, such limitations may be remediable by utilization of a phenomenon in which waves having a phase velocity greater than the phase velocity of a medium radiate energy into the medium at a given angle (Cherenkov angle). There has heretofore been known technology for generating second harmonic of Cherenkov radiation by forming a non-linear optical organic material into fiber form and inputting a coherent primary light thereto, as reported in the Nonlinear Optical Materials - Extended Abstract, 1985, p. 97 and disclosed in US-3 297 875. The radiant beam, however, has the shape of a cone and cannot be focused into a good spot, presenting a problem in utilization of the radiant beam, especially in light of the fact that only a small fraction of total laser energy forms the radiant beam. Generation of second harmonics by Cherenkov radiation from waveguides using lithium niobate (LiNbO₃) as a substrate, has been reported in CLEO ′87 Technical Digest, pp. 198. A device based on the same principle is disclosed in Patent Abstracts of Japan of JP-A-61-189 524. This method has many advantages in that the secondary light having a wavelength one-half that of the fundamental light can be generated. This method simultaneously maintains a high conversion efficiency. Phases of the primary and secondary light also have different wavelengths which can be matched relatively easily. With this method, however, a radiation mode is established from a narrow line-shaped waveguide into the substrate, whereby the light diverges in the direction of radiation while the light is collimated in parallel in the direction at right angles thereto. Accordingly, the wave front is distorted. Therefore, the light cannot be focused to a point maintaining diffraction limited wave front accuracy, which is a fatal defect that hinders the application to optical recording or reading, such as optical disks or laser printers. Such an earlier method is shown in Fig. 4. Therein, the substrate of the waveguide consisted simply of a block having a flat surface. The beam passing through the substrate develops large aberration, but no means for correcting the aberration had ever been employed. Further devices for generating harmonic light are described in EP-A-0 352 751 and EP-A-0 342 523 which are prior art under Art. 54(3) EPC. In these devices the harmonic light is generated by Cherenkov effect in the cladding layer of an optical fiber. The harmonic light beam has a ring-shaped cross section which is difficult to focus on a single point. Summary of the Invention It is therefore an object of the present invention to provide an optical system which generates a collimated beam of harmonic light which can be focused onto a small point. This object is solved by the system each of claims 1 and 2. The subclaims are directed to preferred embodiments of the invention. According to an implementation of the present invention, an optical system is provided in which an abberation correction function or a beam shaping function is given to the waveguide substrate in order to improve the flatness of the wave front formed by the beam that is emitted, and to focus the beam through a lens into a spot of a diffraction limit. That is, the second harmonic Cherenkov beam obtained from the waveguide made of non-linear optical material has the wave front, i.e., has an equiphase front of wave motion, that assumes a conical shape with the fiber as a center line. The presence of ray at right angles with the conical wave front is apparent from Malus's theory. However, it is desirable to have these rays travel in parallel with each other along the center lines of the fibers. First means therefor is based on Snell's law that applies when a ray of light passes through a boundary of media having dissimilar indices of refraction. That is, a first embodiment of the present invention uses a semi-conical prism, as will be described in more detail below, to convert a conical wave front into a parallel plane wave. Such a means is suitably applied to the waveguide type Cherenkov beam which appears as if the fiber-type Cherenkov beam is divided into two. A second embodiment of the present invention applies the law of reflection, in contrast with the application of refraction utilized above. That is, the substrate is fashioned such that it has a totally internal reflective surface with a semi-conical shape, expanding in the direction in which the ray of light travels. Further, the angle relative to the center line of the conical axis is so defined that the ray of light is totally reflected, and all becomes in parallel with the waveguide. A third embodiment of the present invention provides that the second harmonic Cherenkov beam generating portions formed in the waveguide substrate are arranged in a plural number. Thus far only a single one was arranged. The lasing phases among the Cherenkov beams are then coupled together to obtain coherent beams. This embodiment provides an even greater optical output. That is, by the first or the second or the third embodiment, it is made possible to realize a second harmonic optical source capable of focusing a ray into a single spot, close to the diffraction limit, yet maintaining a large output. To obtain Cherenkov rays in the form of parallel beams according to the first embodiment of the present invention, i.e., according to the law of light ray refraction, Snell's law should be taken into consideration on a plane that includes waveguides for generating Cherenkov beams. If a Cherenkov beam angle is denoted by α, an incident angle of beam and refraction angle at the clad face are denoted by i and j, a refractive index of the clad is denoted by n, and a vertical angle of the cone is denoted by Φ as shown in Fig. 2 or 3, there holds the following relationship according to Snell's law, i.e., n(sin i) = sin j Furthermore, the vertical angle Φ of the cone is found from the following relationship, Φ = 90° - j under the condition where the ray of light after refracted becomes in parallel with the waveguide, i.e., Φ = 90° - Tan⁻¹[sin α/(cosα - 1/n)] Conversely, if the vertical angle Φ of the cone is determined as described above, the Cherenkov beam becomes a parallel beam after the refraction and can be focused upon one point. To obtain Cherenkov rays which are all in parallel according to the second embodiment of the present invention, i.e., according to the law of reflection of light beam, on the other hand, the law of reflection is applied on a plane that includes the waveguides. As shown in Fig. 5, the beam radiated at a Cherenkov angle α is totally reflected by the back face of the waveguide's substrate to satisfy the law of reflection i′ = j′. In this case, the back face is formed in a conical shape, and the angle τ subtended by the vertical angle of the cone, i.e., subtended by the axis of the waveguide and by the generatrix is selected to be: τ = i′ = j ′ = ²/₂α whereby the reflected rays of light all become parallel with the waveguides. With the substrate being shaped as described above, there are obtained parallel plane waves which can be focused through a lens into a point maintaining a diffraction limit. Brief Description of the Drawings The invention will be described in conjunction with the drawing which illustrates the preferred embodiments of the invention and comparative examples, and wherein: Fig. 1 is a diagram illustrating a comparative example, wherein Fig. 1(a) is a side view and Fig. 1(b) is a view showing the appearance of a cone prism; Fig. 2 is a diagram illustrating a comparative example, wherein Fig. 2(a) is a side view and Fig. 2(b) is a view showing the appearance of a fiber clad portion; Fig. 3 is a diagram illustrating an embodiment of the present invention, wherein Fig. 3(a) is a side view and Fig. 3(b) is a view showing the appearance of a waveguide substrate; Fig. 4 is a diagram illustrating a conventional example of a Cherenkov wave generator; Fig. 5 is a diagram illustrating an embodiment of the present invention, wherein Fig. 5(a) is a side view, Fig. 5(b) is a front view and Fig. 5(c) is a view showing the appearance of a waveguide substrate; Fig. 6 is a diagram illustrating embodiments of the present invention, wherein Fig. 6(a) is a side view, Fig. 6(b) is a plan view and Fig. 6(c) is a view showing an example of employing fibers or waveguides; Fig. 7 is a diagram illustrating an embodiment of the present invention, wherein Fig. 7(a) is a plan view and Figs. 7(b), 7(c) and 7(d) are views illustrating a phase correcting portion on an enlarged scale; Fig. 8 is a plan view illustrating an embodiment of the present invention; Fig. 9 is a diagram illustrating an embodiment of an optical disk device according to the present invention; Fig. 10 is a diagram illustrating an embodiment of a laser printer according to the present invention; and Fig. 11 is a diagram illustrating an embodiment in which an external cavity resonator is utilized for increasing efficiency of second harmonic generation. Detailed Description of the Preferred Embodiments and Examples Referring now to the figures, it will be seen that Fig. 1 illustrates an example wherein a semiconductor laser 1 serves as a fundamental optical source. The beam from the fundamental optical source is focused through a lens 2, imaged, and communicated to a face of a fiber 3 comprised of a non-linear optical material which has a characteristic response of polarization to an induced electric field. A polarization wave is then induced in the fiber 3. When the phase speed becomes faster than the phase speed of the medium, a polarization wave is radiated as a so-called Cherenkov radiation 4 which is, usually, a blue light having a wavelength one-half that of the fundamental input light, maintaining an angle α. Such a beam diverges with its equiphase surface being in a conical shape. If such a beam is focused through a conical prism 5 having a vertex in the direction in which light travels, the ray of light that has passed through the prism 5 becomes parallel in the direction of the fiber 3, converted into a parallel plane wave which is focused through a focusing lens 6 into a spot 7 of a diffraction limit. Fig. 2 illustrates a second example, wherein the clad 8 that surrounds the fiber 3 has an increased diameter and is formed contiguously with cone prism 5 as a unitary structure. For example, when α = 16° and Φ = 61°, it is possible that a collimated beam can be acquired. Fig. 3 illustrates a preferred embodiment of the present invention. In this embodiment, waveguide 9 for generating second harmonics is formed on a non-linear optical crystal 10, and a Cherenkov radiation beam is collimated through an optical system. That is, the beam from the semiconductor laser 1 is focused and imaged through a lens 2, and is permitted to be incident upon the incident face of the waveguide 9. The fundamental light generates polarization wave in the waveguide 9 and whereby the Cherenkov beam is radiated maintaining an angle α with a lateral divergence angle ϑ. At this moment, the front (exit face) of the substrate 10 of the waveguide 9 assumes a conical shape that is cut into a half as shown in Fig. 3(b). A vertical angle  of the conical shape (angle subtended by the axis and the generatrix) is calculated by the method described above. Thus, there is obtained a beam 11 that is collimated as a parallel light. Depending upon the sizes of the waveguide 9 and the substrate 10, however, the beam 11 may have an anisotropic distribution of intensities. In such a case, use is made of a single or a plurality of anamorphic prisms 12 to shape the beam, whereby there is obtained a plane wave 13 which has an isotropic intensity distribution, which has neat phases, and which has small aberration. Such a beam can be focused into one point of a diffraction limit. Fig. 4 is a diagram illustrating a conventional example lacking any aberration correction for the Cherenkov radiated second harmonics; i.e., blue light is only obtained. This example cannot be applied to a diffraction limit optical system such as optical disks or laser printers. The aforementioned embodiments were to obtain a parallel beam by utilizing a first point of the present invention, i.e., by utilizing the law of refraction of rays. Described below is an embodiment for collimating the Cherenkov beam by the law of reflection. That is, as shown in Fig. 5, the Cherenkov beam is once totally reflected by the back face of the waveguide substrate 10. The inclination of the back surface is then so determined that the ray of light reflected becomes parallel with the waveguide 9. As this holds true for all Cherenkov beams, the reflected beams must become parallel in all cross sections that include the waveguides 9. For this purpose, therefore, the outer face must have a conical shape as shown in Fig.5(a). In this case, the vertical angle of the cone (angle subtended by the axis of the cone and the generatrix) τ is determined to be τ = i′ = j′ = α/2 under the condition where the direction of the waveguide 9 is in parallel with the reflected beam. According to this embodiment, furthermore, the distribution of intensities can also be shaped. By defining the length of the waveguide 9 and the radius of the cone, it is possible to form a beam 14 that it has nearly an isotropic intensity distribution. The beam shaping is in no way limited to this method only, but the method of utilizing the prism described with reference to Fig. 3 can also be applied. Described below is a method of increasing the optical output of the secondary harmonic Cherenkov beam that is formed by collimating the thus obtained highly parallel plane waves. This forms another aspect of the present invention. That is, if the waveguide or the fiber consists of a single strip, it is limitated in wavelength conversion ability. Therefore, the waveguides are arranged in a plural number. Instead, it can be contrived to broaden the width of the waveguide. In this case, however, the transverse modes of high orders are also generated in the waveguide, and there is formed a non-uniform far field of the Cherenkov beam, making it difficult to obtain a single mode. Therefore, the waveguides for holding the single mode are arranged in a plural number to obtain an optical output that is increased by their number. A preferred embobiment of the present invention will now be described with reference to Fig. 6. That is, a beam from a semiconductor laser 1 is focused through a lens 2 to form an image which is then permitted to be incident on the waveguides 15 arranged like an array. In this case, the waveguide substrate 10 has a conical front face or outer face as shown in Figs. 3 or 5. Such fabrication makes it possible to obtain a second harmonic output greater than that of when a single waveguide is used. It need not be mentioned that enhanced effects are obtained even when semiconductor lasers are arranged like an array to form a fundamental optical source having a large output. In the example of Fig. 6(c) fibers are also arranged like an array. In either case, the phases must be coupled among the waveguides or fibers. Accordingly, the waveguide array or fiber array generates second harmonic Cherenkov beams coherently relative to each other. Therefore, consideration must be given also to the case where the laser oscillations from the fundamental semiconductor laser array have phases that are deviated by π relative to the neighboring ones. In such a case, a phase corrector 16 is installed between the semiconductor laser array and the waveguide array for generating secondary harmonics, as shown in the embodiment of Fig.7(a) or (b). In alternative embodiments, the phase corrector 17 or 18 is formed on the face of the waveguide array as shown in the embodiment of Figs. 7(c) or 7(d). This functions to completely render uniform the phases of the laser beam incident upon the waveguide array. This makes it possible to render quite uniform the phases of second harmonics in the obtained Cherenkov beam. Furthermore, if the magnification (reduction ratio) of the lens 2 for forming the image is set to, for example, 1/2, the power density of the incident beam upon the waveguide array becomes four times as great, making it possible to obtain a high wavelength conversion efficiency, which is a feature of this embodiment. With reference to Fig. 8, the semiconductor laser array 1 is directly coupled to the waveguide array in order to reduce the size of the whole second harmonic generator. Fig. 9 illustrates an embodiment in which the beam of large output and high quality obtained as described above is used as an optical source for recording and/or reading on or from an optical disk. That is, the beam from a second harmonic generator (source of blue light) 19 according to the present invention is permitted to pass through a beam splitter 20 and a mirror 21, and is focused into a tiny spot through a focusing lens 22 to record pits onto an optical disk 23. At this moment, the fundamental semiconductor laser is directly modulated in accordance with modulated signals to be recorded by a signal circuit 24. The pits recorded onto the optical disk 23 are read as intensity changes of light reflected from the optical disk. The light is reflected by the beam splitter 20, and is processed by a reproduced signal processing circuit 25. Fig. 10 illustrates an embodiment in which the second harmonic generator 19 according to the present invention is used as an optical source for a laser beam printer. That is, the beam directly modulated by the recording signal circuit 26, is scanned by a polygonal mirror 27, and focused into a spot through a scanning lens 28 to scan on the photosensitive drum 29. A negative image formed on the photosensitive drum is transferred by a printing process onto a paper to effect the high-speed printing. Accordingly, the present invention teaches a system which deals with a Cherenkov-type waveguide that generates second harmonics of a high conversion efficiency utilizing high optical power density and that enables phases to be easily adjusted. There is realized a source of short-wavelength coherent light in which a focusing optical source is formed as a unitary structure, such as was not possible thus far, that is small in size, that enables the intensity of light to be directly modified, and that generates coherent light of short wavelengths which is highly planarly collimated. If such a compact source of short wavelength light that can be directly modulated is used as an optical source for optical disk as shown in Fig. 9, the recording and reproduction can be effected with a wavelength one-half the conventional semiconductor laser wavelength. Therefore, the density for recording data becomes four times as great, or the light output that is required decreases to one-fourth. If the optical source is applied to the laser printer that is shown in Fig. 10, the printing can be carried out in maintaining a resolution which is 4 times as high as that of conventional art. Sensitivity of the photosensitive drum increases, too. Therefore, the printing can be carried out under favorable conditions such as at high printing speeds. The optical source of short waves according to the present invention finds a wide range of applications including not only the aforementioned field of data processing but also the fields of displays and color printers. The conventional technology for generating second harmonics involves a problem in that the non-linear optical crystal is optically damaged. This stems from the fact that too much attention is given to a high conversion efficiency into the second harmonic and whereby the density of the fundamental light power becomes too great in the waveguide. Therefore, as optical damage develops in the waveguide or in the substrate, the light is absorbed in increased amounts and the output of the second harmonic decreases. According to an embodiment of the present invention, however, the waveguides are arranged in a plural number in the form of an array, and the density of light power is suitably decreased to eliminate the problem of optical damage. Turning now to Fig. 11, an embodiment providing improved efficiency in generation of a second harmonic wave will be described. It is generally well known that an external cavity resonator for a fundamental beam is effective for increasing efficiency of second harmonic wave generation. Such resonators are readily adaptable to the present invention. Fig. 11(a) illustrates a Fabry-Perot resonator 30. The resonator 30 is suitably either of the single or plural monolithic type. Fig. 11(b) illustrates incorporation of a Distributed Feedback type resonator 31 in a non-linear optical crystal. Combinations of the type illustrated in Fig. 11 will provide a good quality wavefront SHG beam with higher power. The increased power allows for higher data rate optical recording systems in such applications as optical discs and laser printers. The invention has been described with reference to the preferred embodiments. Obviously, further modification will be apparent upon a reading and understanding of this specification. It is intended that all such modifications be included insofar as they come within the scope of the claims.

Claims: An optical system for generating harmonic light, comprising a substrate and an optical waveguide (9) having non-linear optical characteristics and being formed on said substrate (10) for generating the harmonic light as Cherenkov radiation which is emitted into the substrate and exits the substrate at a predetermined surface thereof, and means (1, 2) for directing coherent light into the waveguide (9) to generate harmonic light therein, characterised in that the exit face of the substrate (10) has a semi-conical shape adapted to collimate the second harmonic light by refraction into a parallel beam (11). An optical system for generating harmonic light, comprising a substrate and an optical waveguide (9) having non-linear optical characteristics and being formed on said substrate (10) for generating the harmonic light as Cherenkov radiation which is emitted into the substrate, and means (1, 2) for directing coherent light into the waveguide (9) to generate harmonic light therein, characterised in that the portion of the substrate (10) which is opposite to the waveguide (9) has a semi-conical surface, the cone axis being aligned with the axis of the optical waveguide, said portion being adapted to collimate the second harmonic light by reflection into a parallel beam (14). The system of claim 1 or 2, characterised in that said harmonic light is second harmonic light. The system of any of claims 1 to 3, further comprising beam shaping means (12) for shaping said parallel beam of harmonic light (13) so as to have an isotropic intensity distribution. The system of any of claims 1 to 4, characterised in that a plurality of waveguides (15) are formed on the substrate (10). The system of claim 5, characterised in that said plurality of waveguides (15) are provided with phase correcting means (16, 17, 18). The use of the system of any of claims 1 to 6 for generating a light beam for writing/reading on or from an optical disc (23) or a photosensitive printer drum (29).

Inventors: TATSUNO KIMIO, TATSUNO, KIMIO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1989
Main classification: G02F1
IPC codes: G02B6, G11B7, G02F1