Optical device and method of manufacturing the same

Abstract: At least one convex arcuate face (or concave arcuate face) is formed by photolithography on a photoresist layer formed on a surface of an optical device constituted from an optical material, and the surface of the optical device and the photoresist film are etched continuously or stepwise to form on the surface of the optical device at least one convex arcuate face (or concave arcuate face) similar to the convex arcuate face (or concave arcuate face) of the photoresist film. In case the optical material is polished into a mirror face, a photoresist film is formed on the polished face, and the polished face and the photoresist film are etched uniformly. An optical device obtained in this manner is used for a laser oscillator, a monolithic laser system, a non-linear optical device, a microlens, a ring laser and so forth. On the other hand, a plurality of reflecting portions are provided on each of a pair of opposing faces of a light transmitting optical material, and a reflecting portion of at least one of the faces is formed from a convex arcuate face formed on the surface of the light transmitting optical material. A zigzag optical path is formed between the reflecting portions of the opposing faces of the light transmitting optical material such that it couples the reflecting portions alternately to each other, thereby to obtain an optical device having a long optical path. The optical device is employed as a higher harmonic wave converting device for light of a high efficiency for a short wavelength laser apparatus.

Description: BACKGROUND OF THE INVENTION This invention relates to an optical part or an optical device, and particularly to an optical part or an optical device having an optical material on which minute, generally spherical or aspherical convex or concave arcuate faces are formed and a process of producing the same. Further, the present invention relates to an optical device having an optical path of a great length such as, for example, a higher harmonic wave converting device and a short wavelength laser apparatus employing such optical device. Conventionally, as a laser resonator, a laser resonator is known of the structure wherein a pair of concave reflecting mirrors are disposed in an opposing relationship to each other on the opposite sides of a laser medium. One of the reflecting mirrors is a total reflecting mirror while the other reflecting mirror is a mirror which passes part of laser light therethrough. In order to miniaturize such a laser system, it is necessary to make the spacing occupied by the reflecting mirrors as small as possible. Further, as miniaturization of systems proceeds, it is necessary to decrease the distance between the reflecting mirrors. Meanwhile, non-linear optical devices are used which convert a frequency of light making use of a nonlinear mutual action of light waves in a substance. The non-linear mutual action may be, for example, production of a second harmonic wave, optical parametric oscillation, production of a difference frequency or the like. Such an optical device is constituted from an optical resonator consisting of two such concave reflecting mirrors as described above, and an optical material in the form of non-linear optical crystal placed between the concave reflecting mirrors. A microlens, which is one of optical parts, is used for an optical communication system or optoelectronics. Since light emerging from a laser or an optical fiber spreads at the angle of 10 to 40 degrees or so, the microlens is used to convert such light into parallel light or further converge such light into a small spot. As one of methods of producing a microlens, there is a method wherein a mask pattern having a necessary circular opening is formed on a glass substrate using the technique of photolithography and ion exchanging is performed through the opening so that the refractive index of a portion of the glass substrate corresponding to the opening is made different from that of the other portion of the glass substrate. While various optical devices are described so far, if such optical devices can be manufactured by forming a convex arcuate face integrally on an end face of an optical material, then this is very advantageous for miniaturization of an apparatus and so forth. By the way, if it is attempted to form a spherical or aspherical convex arcuate face on a surface of an optical material, one of most popular methods is a method by polishing. Accordingly, it is possible to work only one or several works, and consequently, optical devices cannot be manufactured in a mass at a low cost. Further, where such polishing is involved, there is a problem that, from a physical restriction, a plurality of convex arcuate faces cannot be formed in the proximity of each other on a surface of an optical material. A Fresnel lens is known as an example of an aggregate of small lenses. In this instance, since an electron beam picture drawing apparatus is employed for production of a Fresnel lens, the equipment is very expensive and the mass productivity is low. Further, while it is possible to manufacture, by molding, a microlens on which a plurality of convex arcuate faces are provided, the material of the microlens is limited to such material that is suitable for molding. Accordingly, there is a problem that it is impossible to manufacture a microlens using such a material as a single crystal material or a high melting point amorphous material. By the way, since generally a laser is limited in output wavelength, there is a method wherein laser light is converted into coherent light of another wavelength making use of a non-linear optical phenomenon in order to obtain laser light of a shorter wavelength. As a representative example of the same, there is a method wherein laser light is converted into a higher harmonic wave by means of a higher harmonic wave generator (SHG). This depends upon the fact that, if laser light of a frequency ω o is introduced into crystal of a non-linear medium, then light of the frequency 2ω o is outputted from the crystal. Such higher harmonic conversion has been attempted in accordance with various methods, and it is known that a higher harmonic output thus converted increases in proportion to an input power to the second power or an internal power to the second power of a resonator and a length to the second power of the non-linear medium (however, at an optimum focus, the higher harmonic wave output increases in proportion to a length of the non-linear medium). Accordingly, in order to increase the higher harmonic wave output, it is necessary to increase the internal power or the length of the non-linear medium. As a method for higher harmonic conversion which is conventionally employed, there is a method of direct conversion wherein non-linear crystal is cut into a piece, and low reflection coatings are applied to the polished opposite end faces of the non-linear crystal piece, and then exciting light is irradiated upon the non-linear crystal piece. With the present method, direct conversion making use of a semiconductor laser is possible, but since generally it is difficult to obtain a long non-linear crystal piece, there is a problem that the available higher harmonic wave output is very low. Further, there is another method wherein a non-linear medium is disposed in a laser resonator. With the present method, since the internal power of the resonator can be made equal to 100 times or so the intensity of incidence light readily, a comparatively high higher harmonic output can be extracted. However, there are problems that the system is complicated and that a separate modulator is required because direct modulation by the semiconductor laser is not performed. Further, as a method which efficiently raises the incidence power and artificially satisfies a phase matching requirement, there is a method wherein exciting light is enclosed by means of a waveguide structure to effect higher harmonic conversion. The method, however, has problems that technical difficulty is involved in production of the waveguide and besides that it is very difficult to introduce exciting light into the waveguide and efficiently extract a higher harmonic wave from the waveguide. Also, there is a problem that, since it is difficult, from a restriction in manufacture, to obtain a symmetrical structure with respect to an optical axis and consequently it is actually difficult to extract a higher harmonic wave of a single mode, it is limited in application. As described so far, with the methods proposed till now, the output is too low, and if the case is considered wherein an optical device is used as a light source, for example, for an optical disk, then the situation is such that it is difficult to provide a light source device which has a higher harmonic wave output of 1 mW or so and can effect simple modulation. By the way, a solid-state laser which is longitudinally excited by a laser diode can present a high gain due to a wide spectrum and pumping having a spatial characteristic. Various laser devices of the high output type (including multi-facet pumping devices, LD multiplexing devices and fiber bundle pumping devices) have been developed till now. However, in order to scale up the longitudinal excitation to increase the output power, generally it is necessary to make a cavity large so that the laser device may stand a higher basic mode. Further, if the longitudinal excitation is scaled up, then the gain is decreased due to double refraction or aberration which is caused by heat. On the other hand, as a method for scaling up the longitudinal excitation while keeping the cavity compact, there is a method wherein, making use of a thermal lens effect which is caused by exciting light introduced into a solid-state laser medium, a pair of flat mirrors are disposed on the laser medium so that multi-array excitation may take place (Oka, M. et al . CLEO '91, p.40). However, this method has problems that, since it makes use of a thermal lens effect, the threshold level in laser oscillation is high and the oscillation efficiency is low. According to one aspect of the present invention, there is provided an optical device constituted from a light transmitting optical material having: a first face and a second face opposing each other; and a plurality of first reflecting portions provided on said first face and a plurality of second reflecting portions provided on said second face individually in an opposing relationship to said first reflecting portions, to provide a zigzag optical path between said first reflecting portions and said second reflecting portions alternately coupling said first reflecting portions and said second reflecting portions to each other; characterised in that at least one reflecting portion of at least one of said two pluralities of reflecting portions is obtained by coating a reflecting film on a convex arcuate face on the appropriate first or second face. The present invention further provides a short wavelength laser apparatus which employs such an optical device. Preferably, the convex arcuate face is a generally spherical face or aspherical face, for example, a face of an ellipsoid of revolution. In this instance, the spherical face or aspherical face may have distortion wherein the radius of curvature in a radial direction variers continuously or stepwise. The present invention also provides a method of manufacturing an optical device as defined above, including the steps of: forming at least one convex arcuate face in a photoresist film formed on the surface of the light transmitting optical material by exposure based on photolithography and development on said photoresist film; etching the surface of the light transmitting optical material and the photoresist film to form, on the surface of the light transmitting optical material, said at least one convex arcuate face similar to the convex arcuate face of the photoresist film, and coating a reflecting film on surfaces of the face on which the at least one convex arcuate face is formed and another face opposing to the face on which the at least one convex arcuate face is formed to provide said reflecting portions, wherein in said step of forming at least one convex arcuate face by exposure based on photolithography, when the photoresist film is exposed to light of a circular or elliptic pattern, the intensity of exposure light is varied gradually or stepwise from the centre toward an outer periphery of the pattern. There is alternatively provided a method of manufacturing an optical device as defined above, including the steps of: forming a pattern by photolithography on a photoresist film which is formed on a surface of the light transmitting optical material and has a generally flat and smooth upper end face, to form at least one circular column-shaped or elliptic column-shaped photoresist film; heat treating the photoresist film to deform the generally flat upper end face of the photoresist film into at least one convex arcuate face; etching the surface of the light transmitting optical material and the deformed photoresist film to form at least one convex arcuate face similar to the at least one convex arcuate face of the photoresist film on the surface of the light transmitting optical material; and coating a reflecting film on surfaces of the face of the light transmitting optical material on which the at least one convex arcuate face is formed and another face of the light transmitting optical material opposing the face on which the at least one convex arcuate face is formed, to provide said reflecting portions. In the foregoing, preferably etching is performed by dry etching. Gas to be used in etching can be determined suitably depending upon an optical material employed. Conditions of etching depend upon a profile of a convex arcuate face to be formed on a surface of an optical material. In the case of dry etching, some distortion can be provided to the convex arcuate face by changing the sectional shape of the convex arcuate face to be formed on the surface of the optical material (the sectional shape in a direction perpendicular to the surface of the optical material) by continuously or stepwise changing, at any time during etching, the kind, amount or flow rate of gas used, the high frequency output, the intensity of a magnetic field for enclosure, the accelerating voltage for gas ions, the etching time or the like. As the optical material, laser materials such as Nd:YAG, Nd:YAB, Nd:Y 3 Al 5 O 12 , Nd: YLiF 4 , Nd:YVO 4, Nd:La 2 Be 2 O 5 and Nd:Y 3 Al 3 (BO 3 ) 4 , non-linear materials such as KNbO 3 , LiNbO 3 and KTiOPO 4 , amorphous materials such as BK7, composite quartz and glass and crystal materials such as quartz, calcite, silcon and GaAs can be listed. As a photoresist, preferably a photoresist obtained by polymerization of a diazo photosensitive material with a phenol resin generally called positive type photoresist is employed. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagrammatic view showing different steps of an example of a manufacturing method for an optical device of the present invention; Fig. 2 is a schematic diagrammatic view showing different steps of another example of a manufacturing method for an optical device of the present invention; Fig. 3 is a schematic diagrammatic view showing different steps of a still further example of a manufacturing method for an optical device of the present invention; Fig. 4 is a diagram illustrating comparison between the speeds of etching of a photoresist film and a glass substrate when the accelerating voltage is varied as a dry etching condition; Figs. 5 and 6 are diagrammatic sectional views showing examples of optical devices which have a convex arcuate face on a face thereof and have a zigzag optical path; Fig. 7 is a diagrammatic sectional view showing another example of an optical device which has convex arcuate faces on both faces opposing each other and has a zigzag optical path; Figs. 8, 9 and 10 are diagrammatic sectional views showing further examples of optical devices which have reflecting portions each in the form of a flat plate and have a zigzag optical path (but which are not embodiments of the invention); Fig. 11 is a diagrammatic sectional showing an optical device which has a reflecting portion formed from prisms and has a zigzag optical path (but which is not an embodiment of the invention); Figs. 12 and 13 are diagrammatic sectional views of optical devices wherein two materials are joined to each other and convex arcuate faces are formed on one and both of the faces opposing each other, respectively; Fig. 14 is a perspective view showing an example of an optical device having a zigzag optical path; Fig. 15 is a schematic view showing an example of short wavelength laser apparatus which employs an optical device of the present invention; DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of optical devices of the present invention are shown in the figures. Referring to Fig. 15, reference numeral 10 denotes a laser resonator, and 12 a generally spherical convex arcuate face formed on an end face of the laser resonator 10. Sectional views of an optical device having a zigzag optical path of the present invention are shown in Figs. 5 to 13. A short wavelength laser apparatus which employs such an optical device as a higher harmonic wave converting device is shown in Fig. 15. In the following, a method of manufacturing an optical device of the present invention and a laser apparatus which employs the optical device will be described in connection with several examples with reference to the drawings. Normally, a plurality of convex arcuate faces 12 or the like are formed on an optical material having a somewhat great area, and they are used as they are as optical devices or they are cut to form an optical device. 1. Optical Device having Zigzag Optical Path Of the optical devices described hereinabove, the optical device which is characterized in that, as shown in Figs. 5 to 7, it is formed from a light transmitting optical material having a first face and a second face opposing to each other and has a plurality of first reflecting portions provided on the first face and a plurality of second reflecting portions provided on the second face individually in an opposing relationship to the first reflecting portions and further has a zigzag optical path which is provided between the first reflecting portions and the second reflecting portions and alternately couples the first reflecting portions and the second reflecting portions to each other, and each of at least ones of the first reflecting portions and the second reflecting portions is a reflecting portion formed by coating a reflecting film on a convex arcuate face, has an optical path of a great length and can be used, for example, as a higher harmonic converting device. A window for passing light therethrough is provided at each of an end portion of the first face and an opposite end portion of the second face. One of the windows will be hereinafter referred to as incidence window, and the other will be hereinafter referred as outputting window. Light introduced into the light transmitting optical material through the incidence window passes the zigzag optical path described above and outputs through the output window. The input window and the output window may be provided at the opposite end portions of one of the faces (Fig. 9). As an optical device having a zigzag optical path, an optical device can be listed wherein a convex arcuate face is formed on a first one of a pair of opposing faces of a light transmitting optical material and the second face is formed as a flat face as shown in Figs. 5 and 6. Further, such junction type devices as shown in Figs. 12 and 13 can be listed. Further, a higher harmonic wave converting device can be listed which is characterized in that, as shown in Fig. 7, a first convex art face is provided on one of a pair of opposing faces of a light transmitting optical material while a second convex arcuate face is provided on the other face, and coatings are provided on both faces and a window is formed at an end portion of each of the faces. Preferably, each of the convex arcuate faces is a generally spherical face or aspherical face, for example, a face of an ellipsoid of revolution. In this instance, the spherical face or aspherical face may have distortion wherein the radius of curvature varies continuously or stepwise in a radial direction. Upon formation of the convex arcuate faces, the error in formation of the convex arcuate faces can be corrected by making a plurality of sets of convex arcuate faces having radii of curvature which are different by a small amount from each other, effecting laser oscillation or higher harmonic conversion using a laser oscillator and selecting a set of convex arcuate faces with which the oscillation efficiency or the higher harmonic conversion efficiency is the highest. The diameter, the radius of curvature and so forth of the convex arcuate faces are suitably set depending upon an object of use, a wavelength and so forth of the optical device. Formation of the convex arcuate faces may be performed similarly as described hereinabove. While the first reflecting portions and the second reflecting portions described above are constructed, preferably for the maintenance of a light converging condition and the enhancement of the power intensity of the inside, such that a convex arcuate face such as a spherical case is formed at least on one face and part or all of a reflecting portion is formed from a reflecting mirror provided by the convex arcuate face (Figs. 5 to 7), the two reflecting portions may be in the form of flat faces (Figs. 8 to 10). Further, a prism may be provided as a reflecting portion in place of a convex arcuate face (Fig. 11). In order to form a prism on an optical material, at an exposing step wherein a photoresist film is exposed to light of a circular or elliptic pattern in the photolithography method described above, the distribution of the exposure light intensity should be varied in accordance with a profile of the prism. The reflecting portions and windows can be formed by coating a total reflecting film for light on a surface of a light transmitting optical material at locations at which the respective reflecting portions are to be provided while no coating is provided at locations at which the windows are to be provided (Figs. 5, 10 etc.). Or else, they can be formed by coating a total reflecting film for light on an entire face of each of the first and second faces and then removing the coatings at locations of the windows. Further, coating may be performed only for each of the reflecting portions (Figs. 8 and 9). As a method of such coating, a method wherein two kinds of thin films having different refractive indexes are layered, by vacuum vapor deposition or sputtering, alternately to such thicknesses with which the optical lengths at an aimed reflecting wavelength correspond to the 1/2 wavelength can be listed. As a method of removing the coatings, a method wherein coating is performed in a condition wherein, for example, a photoresist film is formed at each window portion and the coatings are removed together with the photoresist film or another method wherein a photoresist film is formed after application of a coating and then exposure and development are performed to remove the photoresist at each window portion, whereafter the exposed coating is removed by dry etching, can be listed. Subsequently, an example of coating and formation of an incidence window and an output window will be described in detail. Photoresist is spin coated on a surface of a light transmitting optical material, and pre-baking is performed to form a photoresist film. Subsequently, a photomask having a circular light intercepting pattern is closely contacted with a surface of the photoresist and ultraviolet rays are irradiated upon the photoresist film to effect close contact exposure of the photoresist film. The photoresist film exposed to ultraviolet rays is developed and then rinsed with pure water, whereafter it is dried. The photoresist film is first dried for 30 minutes in a clean oven at 100 °C, and then, post-baking is performed for an hour in another clean oven at 175 °C. Dry etching of the light transmitting optical material wherein a convex arcuate face is made of the photoresist in this manner is performed using a dry etching equipment to transfer the convex arcuate face made of the photoresist to the surface of the light transmitting optical material. Coating on the surface and formation of a window in the light transmitting optical material on which the convex arcuate face or no convex arcuate face is formed in this manner is performed, for example, in the following manner. Prior to coating, photoresist is spin coated on the surface of the light transmitting optical material and pre-baking is performed to form a photoresist film, and then the photoresist is left only at each window portion by close contact exposure and developing processing. In this condition, TiO 2 and SiO 2 are alternately vapor deposited by an electron beam by means of an electron beam vapor depositing apparatus. Preferably, a coated film is such a film that reflects almost all of light of frequencies between a basic wave and a higher harmonic wave. After the specimen is taken out of the vapor depositing apparatus, it is soaked in acetone and washed using an ultrasonic washing machine to remove the photoresist and the coated film deposited on the photoresist film to form a window. Further, formation of a window is possible not only by the so-called lift-off method described above but also by etching. A method can be listed wherein, after coating is performed, spin coating and pre-baking are performed for the photoresist to form a photoresist film, whereafter exposure, development and water washing are performed using a photomask to expose the window portion through the photoresist film, and then, part or all of the coated film is removed by dry etching to form a window. If coating for prevention of reflection is performed after formation of the window portion, then an element of a further high performance can be manufactured. Light introduced into the incidence window of the optical device obtained in this manner is repetitively reflected alternately by the two reflecting portions such that it advances in a zigzag pattern and then outputs through the output window. Since, due to such an action, it is possible to assure a great optical path length and a beam can be successively converged by a light converging effect of the concave mirrors, the optical device can be utilized as a higher harmonic wave converting device for light, a laser oscillator such as a slab laser and so forth. As the light transmitting optical material for use for an optical device having such a great optical path length, laser materials such as Nd:YAG, Nd:YAB, Nd:Y 3 Al 5 O 12 , Nd:YLiF 4 , Nd:YVO 4 , Nd:La 2 Be 2 O 5 and Nd:Y 3 Al 3 (BO 3 ) 4 , non-linear materials such as KNbO 3 , LiNbO 3 and KTiOPO 4 and amorphous materials such as BK7, composite quartz and glass can be listed. Upon production of an optical device of the present invention, a plurality of optical devices can be produced readily at a low cost by cutting a light transmitting optical material after a plurality of sets of convex arcuate faces and windows are formed collectively. 2. Short Wavelength Laser Apparatus Employing Device Having Zigzag Optical Path A short wavelength laser apparatus can be constructed by using a non-linear optical device on which reflecting portions and windows are formed in such a manner as described above as a higher harmonic wave converting device and disposing the higher harmonic wave converting device and an exciting laser oscillator such that laser light may be converged to one of the windows formed on the higher harmonic wave converting device. Since laser light outputted from a laser oscillator is emitted normally with a fixed angle, it is preferable to convert the laser light into parallel light or converge the same into a spot using a focusing lens in accordance with the necessity. Laser light outputted from the exciting laser oscillator is converted into parallel light or converged into a spot by the focusing lens so that it is converged as a basic wave into the incidence window of the higher harmonic wave converting device. The fundamental wave is introduced into the higher harmonic wave converting device through the incidence window and then repetitively reflected by the two reflecting portions of the higher harmonic wave converting device so that it is added into a higher harmonic, and finally, the basic wave and higher harmonic wave are outputted from the output window. Further, by employing a laser diode as the laser oscillator, a short wavelength laser apparatus which is small in size, simple and convenient and can effect direct modulation can be constructed. Example 1 First, a first example of method of manufacturing an optical device of the present invention will be described with reference to Fig. 1. A glass substrate 40 was used as an optical material on which a convex arcuate face 12 was to be formed. Photoresist OFPR800 made by Tokyo Ohka Kogyo Kabushiki Kaisha. was spin coated on a surface of the glass substrate 40, and then the glass substrate 40 was pre-baked to form a photoresist film 42 of 0.6 µm thick ((a) of Fig. 1). Subsequently, using as an exposing apparatus an enlarging machine 50 of the model CF670 made by Fuji Shashin Film Kabushiki Kaisha. in which a FUJINON EX 50 mm lens set to F4 was mounted, the enlarging machine was adjusted such that the size of a pattern formed on an exposure negative 52 and the size of an image of the pattern might have the ratio of 1:1. It is to be noted that, though not shown, an upper lamp house was removed and a cold mirror of the center wavelength of 400 nm was mounted, and then an extra-high pressure mercury lamp was set to a lamp house of the model UIS-5100 made by Ushio Denki Kabushiki Kaisha so that ultraviolet rays were introduced as an exposure light source into the lamp house. The pattern of the negative 52 was a circular pattern. The glass substrate 40 on which the photoresist film 42 was formed was set to a focus position (refer to (b) of Fig. 1), and ultraviolet rays were irradiated upon the glass substrate 40 to form, on the photoresist film 42, an image of the pattern formed on the negative 52, whereafter the photoresist film 42 was developed. Consequently, the photoresist film 42 of a circular shape having a diameter of 100 µm remained on the surface of the glass substrate 40 as shown in (c) of Fig. 1. The surface of the circular photoresist film 42 had a generally spherical shape and the radius of curvature of the surface was about 2 mm. When a lens having a low resolution such as that in an enlarging machine for a photographic film is used, the amount of ultraviolet rays irradiated upon the photoresist film 42 increases from a central portion toward a peripheral portion of each circular pattern. Therefore, if the photoresist film 42 on which images of patterns are formed are developed, then the thickness of the photoresist film 42 remaining on the surface of the glass substrate 40 decreases from a central portion to a peripheral portion of each circular pattern. Accordingly, the surface of the photoresist film 42 after development presents a generally spherical profile. It is to be noted that, when the glass substrate 40 on which the photoresist film 42 was formed was set to a position displaced by 3 mm from the focus position and then similar processing was performed, the photoresist film 42 of a circular shape having a diameter of 60 µm was left on the surface of the glass substrate 40. The surface of the circular photoresist 42 had a generally spherical profile and the radius of curvature of the spherical face was about 1.5 mm. Further, when a white circular shape pattern was photographed in a somewhat defocused condition to obtain a negative film having a circular pattern wherein a circular central portion is black and the photographing density decreases toward an outer periphery and the negative film was used as the negative 52 for circular pattern transfer, a semi-spherical photoresist pattern was formed without particularly effecting a defocusing operation for the glass substrate 40. The glass substrate 40 obtained in this manner and having the circular photoresist film 42 of the diameter of 100 µm formed thereon was dry etched using a dry etching equipment of the model ECR-310 made by Nichiden Aneruba Kabushiki Kaisha. Dry etching conditions were such that, after the dry etching equipment was exhausted to 6.5 × 10 -4 Pa, C 2 F 6 was introduced by 5 SCCM (Standard Cubic Centimeter Minute) into the equipment, and etching was performed for one hour at the high frequency output of 300 W, the enclosing magnetic field of 10 -2 T and the ion accelerating voltage of 500 V. By the dry etching under the conditions, the photoresist film 42 formed on the surface of the glass substrate 40 was etched and disappeared completely. Simultaneously, also the glass substrate 40 was etched so that a convex arcuate face 12 having a generally spherical shape having the diameter of about 100 µm and the radius of curvature of about 4 mm was formed on the surface of the glass substrate 40 ((d) of Fig. 1). The reason why the radius of curvature of the convex arcuate face 12 formed on the surface of the glass substrate 40 and the radius of curvature of the surface of the photoresist film 42 are different from each other is that the etching rates at the glass substrate 40 and the photoresist film 42 are different from each other. Example 2 Subsequently, a second example of method of manufacturing an optical device of the present invention will be described. A glass substrate 40 was used as an optical material on which a convex arcuate face 12 was to be formed. Photoresist OFPR800 made by Tokyo Ohka Kogyo Kabushiki Kaisha. was spin coated on a surface of the glass substrate 40, and then the glass substrate 40 was pre-baked to form a photoresist film 42 of 0.6 µm thick ((a) of Fig. 2). Using a mask aligner of the model QUINTEL Q6000, the photoresist film 42 was exposed to light in a condition wherein the distance between the photomask 60 and the photoresist film 42 was kept equal to 20 µm ((b) of Fig. 2). It is to be noted that, in this instance, a diffuser 62 (DFSQ-50C02-1500 made by Siguma Koki) obtained by sand-blasting of the opposite faces of an optical glass substrate BK7 of 1 mm thick with abrasive grain of alumina of #1500 was placed on the photomask 60 so that diffused light having passed through the diffuser 62 might be irradiated upon the photoresist film 42 on the glass substrate 40 through the photomask 60. In this instance, the photomask 60 used was made of sodium lime glass, 25.8 square cms (4 inches square) and 0.228 centimeters (0.09 inches) thick and had a circular pattern having a diameter equal to or smaller than 100 µm. Subsequently, development of the photoresist film 42 was performed using developer NMD-3 made by Tokyo Ohka Kogyo Kabushiki Kaisha. After development and rinsing with pure water, the photoresist film 42 of a circular pattern having a diameter equal to or smaller than 100 µm remained on the glass substrate 40 ((c) of Fig. 2). The surface of the photoresist film 42 was a convex arcuate face 42a of a substantially spherical shape having the radius of curvature of about 4 mm. In this manner, the circular photoresist film 42 having the generally spherical convex arcuate face 42a was able to be formed by using diffused light with a suitable distance left between the photomask 60 and the photoresist film 42 in this manner. The glass substrate 40 obtained in this manner was dry etched using the dry etching equipment of the model ECR-310 E made by Nichiden Aneruba Kabushiki Kaisha. Dry etching conditions were such that, after the dry etching equipment was exhausted to 6.5 × 10 -4 Pa, C 2 F 6 was introduced by 5 SCCM into the equipment, and etching was performed for 20 minutes at the high frequency output of 300 W, the enclosing magnetic field of 10 -2 T and the ion accelerating voltage of 700 V. By the dry etching under the conditions, the photoresist film 42 formed on the surface of the glass substrate 40 was etched and disappeared completely. Simultaneously, also the glass substrate 40 was etched so that a convex arcuate face 12 having a generally spherical shape having the diameter of about 100 µm and the radius of curvature of about 4 mm was formed on the surface of the glass substrate 40 ((d) of Fig. 2). Example 3 Further, a third example of method of manufacturing an optical device of the present invention will be described. A photoresist film 42 of 15 µm thick was formed on a glass substrate 40, and while the distance between a photomask 60 and the photoresist film 42 was held equal to 8 mm, the photoresist film 42 was exposed to light using the photomask 60 having a circular pattern of the diameter of 1 mm, whereafter development was performed under the same conditions as in the second example so that the photoresist film 42 of a circular profile was left on a surface of the glass substrate 40. The diameter of the photoresist film 42 thus left was 0.8 mm and the radius of curvature of the surface was about 8 mm. The glass substrate 40 was dry etched similarly as in the second example. However, the etching time was 8 hours. A convex arcuate face 12 of a substantially spherical profile having the diameter of 0.8 mm and the radius of curvature of 8 mm was formed on the surface of the glass substrate 40 after etching. The ratio between the etching rates at the photoresist film 42 and the glass substrate 40 can be varied by varying a dry etching condition such as, for example, the amount of gas to be introduced into the dry etching equipment, the accelerating voltage or the like. Consequently, even if the radius of curvature of the convex arcuate face 42a of the photoresist film 42 left on the glass substrate 40 is equal, the radius of curvature of the convex arcuate face 12 of the surface of the glass substrate 40 after dry etching can be varied. For example, when the glass substrate 40 on which the photoresist film 42 having the generally spherical convex arcuate face 42a having the diameter of 100 µm and the radius of curvature of about 4 mm was dry etched in accordance with the following dry etching conditions, the radii of curvature of the generally spherical convex arcuate faces 12 on the surface of the glass substrate 40 obtained were such as described below. Gas Used C 2 F 6 C 2 F 6 Quantity of Gas (SCCM) 3 10 High Frequency Output (W) 300 300 Enclosing Magnet Field (T) 10 -2 10 -2 Ion Accelerating Voltage(V) 700 300 Etching Time 45 min. 75 min. Radius of Curvature of Spherical Convex Arcuate Face on Surface of Glass Substrate (mm) 2.8 5.6 Example 4 Further, a fourth embodiment of method of manufacturing an optical device of the present invention will be described. A glass substrate 40 was used as an optical material on which a convex arcuate face 12 was to be formed. Photoresist OFPR800 made by Tokyo Ohka Kogyo Kabushiki Kaisha. was spin coated on a surface of the glass substrate, and then the glass substrate was pre-baked to form a photoresist film 42 of 8 µm thick ((a) of Fig. 3). Subsequently, by a so-called close contact exposure method wherein a photomask 60 was closely contacted with the photoresist film 42, the photoresist film 42 was exposed to light through the photo mask with a circular pattern of the diameter of 100 µm ((b) of Fig. 3). The photoresist film 42 was developed using developer NMD-3 made by Tokyo Oka Kogyo Kabushiki Kaisha and then rinsed with pure water. The photoresist film 42 of a column-shaped profile of the diameter of 100 µm remained on the glass substrate 40 ((c) of Fig. 3). The glass substrate 40 was placed for 30 minutes in a clean oven of 175 °C. Since the photoresist film 42 was held at a temperature higher than a glass transition point of the material constituting the photoresist film 42, part of it was thermally fluidized so that it was deformed into a liquid drop so that a generally spherical convex arcuate face 42a having the radius of curvature of about 100 µm was formed ((d) of Fig. 3). The heating conditions for the photoresist film 42 must only be a temperature higher than a glass transition point of the material constituting the photoresist film 42 and a time sufficient for part of the photoresist film 42 to be thermally fluidized. The glass substrate 40 obtained in this manner was dry etched using the dry etching equipment of the model ECR-310 E made by Nichiden Aneruba Kabushiki Kaisha. Dry etching conditions were such that, after the dry etching equipment was exhausted to 6.5 × 10 -4 Pa, oxygen was introduced by 5 SCCM into the equipment, and etching was performed for 1 hour at the high frequency output of 300 W, the enclosing magnetic field of 10 -2 T and the ion accelerating voltage of 500 V. By the dry etching under the conditions, the photoresist film 42 formed on the surface of the glass substrate 40 was etched and disappeared completely. Simultaneously, also the glass substrate 40 was etched so that a convex arcuate face 12 having a generally spherical shape having the diameter of about 100 µm and the radius of curvature of about 5 mm was formed on the surface of the glass substrate 40 ((e) of Fig. 3). The etching rate can be varied by varying the ion accelerating voltage, and at the accelerating voltages of 300 V and 700 V, the radius of curvature of the generally spherical convex arcuate face 12 on the surface of the glass substrate 40 was about 10 and 2.5 mm, respectively. Further, when not a circular shape but an elliptic shape is used for a pattern of the photomask 60, the convex arcuate face 12 to be formed finally on the surface of the glass substrate 40 does not have a spherical shape but has a shape of a face of an ellipsoid of revolution, but also the convex arcuate face 12 of such a shape of a face of an ellipsoid of revolution was able to be formed readily by the method of the example described above. Example 5 A convex arcuate face 12 was formed on a surface of a glass substrate 40 by a method similar to that of the Example 3 except that the pattern of the photomask 60 was an elliptic shape having the major diameter of 150 µm and the miner diameter of 75 µm. When the ion accelerating voltage was set to 500 V, the convex arcuate face 12 on the surface of the glass substrate 40 obtained had the shape of a face of an ellipsoid of revolution having the radius of curvature of about 11.3 mm on the major diameter side and the radius of curvature of about 2.8 mm on the minor diameter side. With an optical material other than the glass substrate 40, for example, with such a material as KNbO 3 or Nd:Y 3 Al 5 O 12 , when the etching rate is different from that of the glass substrate 40, a convex arcuate face 12 or a concave arcuate face can be formed readily on a surface of the same by a method similar to those of the Examples 1 to 6 described above. Example 6 The present example is an example wherein it was tested to see in what manner a spherical convex arcuate face of a glass substrate after etching varies when an etching condition is varied continuously during etching using photoresist, on which a spherical face is formed, as a mask. First, a generally spherical convex arcuate face 42a having the radius of curvature of about 100 µm was formed on a glass substrate 40 similarly as in the Example 4 except that the photoresist film 42 was formed with the thickness of 6 µm. Subsequently, the glass substrate 40 on which the false spherical face was formed from the photoresist in this manner was set to the dry etching equipment of the model ECR-310 E made by Nichiden Aneruba Kabushiki Kaisha. and exhausted to 6.5 × 10 -4 Pa, and then oxygen was introduced by 5 SCCM into the dry etching equipment. Thus, etching was performed for one hour and thirty minutes at the high frequency output of 300 W and the enclosing magnetic field of 10 -2 T while decreasing the ion accelerating voltage at the rate of 1 V/minute from 500 V. In this instance, the photoresist disappeared completely, and the profile of the photoresist was transferred to the glass substrate so that a spherical face having some deformation wherein the diameter was about 100 µm and the radius of curvature was 5 mm at a central portion and 4.3 mm at a peripheral portion was produced. Fig. 4 shows a relationship between the accelerating voltage (X-axis) and the ratio between the etching rates at the photoresist OFPR800 made by Tokyo Ohka Kogyo Kabushiki Kaisha. and the BK7 glass substrate in the ECR-310 E equipment when the glass substrate is etched using the photoresist as a mask. As apparently seen from Fig. 4, if an etching condition is varied continuously during etching, arbitrary distortion in a radial direction can be provided to a spherical face of the glass substrate. While, in the present example, the accelerating voltage was continuously varied to control the profile of the spherical face after etching, a similar effect is obtained if not the accelerating voltage but some other parameter such as the gas flow rate or the high frequency output is varied. Further, even if a parameter is varied not continuously but stepwise, a spherical face having stepwise distortion in a radial direction is obtained. When the profile of the spherical face after development and rinsing is an elliptic column-shaped profile, an aspherical elliptic lens which is most suitable for convergency of light of a laser diode can be manufactured by this method. According to this method, an etching condition can be varied during etching to form an aspherical face having an arbitrary degree of distortion from a spherical face. The optical devices on which convex arcuate faces are formed in the Examples 1 to 6 described so far can be used as optical devices having a zigzag optical path by applying a coating to them and providing a reflecting portion and a window portion to them. Example 7 An example of short wavelength laser apparatus wherein an optical device having a zigzag optical path is employed as a higher harmonic wave converting device will be described. An outline of construction of a higher harmonic wave converting device and a short wavelength laser apparatus according to the present example is shown in Fig. 15. Referring to Fig. 15, reference numeral 1 denotes a laser diode SLD7033101 made by SANYO Kabushiki Kaisha, 2 a focusing lens F-L40B made by New Port Co., 3 a plano-concave lens ø = 25 mm, f = 62.9 mm, and 10 a higher harmonic wave converting device according to the present invention obtained by polishing a face a of a single crystal piece of KNbO 3 , forming convex arcuate faces on one face, applying a high reflection coating on the one face and removing part of the coating. Reference numeral 12 denotes a convex spherical face formed on a surface of the light transmitting optical material, 17 an incidence window formed by removing part of the coated film in order to admit a basic wave into the light transmitting optical material, and 18 a higher harmonic wave outputting window formed by removing part of the coated film in order to extract a higher harmonic wave from the light transmitting optical material. A broken line indicates an optic axis. The coating is performed for the face of the light transmitting optical material having the convex spherical faces and another face opposing to the face such that the reflection factor at the wavelength of 860 nm may be 99.95 % and the reflection factor at the wavelength of 430 nm may be 99.9 %. A perspective view of a higher harmonic wave converting device actually produced using KNbO 3 as a light transmitting optical material is shown in Fig. 14. The higher harmonic wave converting device was made in the following manner. Photoresist OFPR800 made by Tokyo Ohka Kogyo Kabushiki Kaisha was spin coated on a surface of the KNbO 3 material having a face a of 10 mm × 10 mm and the thickness of 5 mm, and pre-baking was performed for the photoresist film of 6 µm thick. Then, a photomask having circular light intercepting patterns was closely contacted with a surface of the photoresist film and ultraviolet rays were irradiated upon the photoresist film to effect close contact exposure. In this instance, a plurality of combinations of circular patterns wherein the diameter was varied by small amounts among 104 µm, 102 µm, 100 µm, 98 µm and 96 µm were disposed on the photomask. The sets of the individual diameters correspond to rows of convex spherical faces denoted by 4 to 8 in Fig. 14. The photoresist film exposed to ultraviolet rays was developed for one minute with developer NMD-3 made by Tokyo Ohka Kogyo Kabushiki Kaisha and then rinsed with pure water, whereafter it was dried. The photoresist films each in the form of a round column remained on the KNbO 3 material. The KNbO 3 material was first dried for 30 minutes in the clean oven of 100 °C and then post-baked for one hour in the clean oven of 175 °C. As a result, generally spherical convex arcuate faces were formed due to thermal fluidization. The KNbO 3 material on the surface of which the spherical photoresist was formed in this manner was dry etched using the dry etching equipment of the model ECR-310E made by Nichiden Aneruba Kabushiki Kaisha to transfer the convex spherical faces produced from the photoresist. In this instance, etching conditions were: etching gas, oxygen 5SCCM; microwave input 300 W; ion enclosing magnetic field 10 -2 T; and ion accelerating voltage 325 V. The radii of curvature of the convex spherical faces produced in this manner were 10.44 mm with the spherical faces produced from the pattern of the diameter of 104 µm, and 10.26 mm, 10.12 mm, 9.96 mm and 9.80 mm with the spherical faces produced from the patterns of the diameters of 102 µm, 100 µm, 98 µm and 96 µm, respectively. A high reflection coating consisting of TiO 2 and SiO 2 was applied to each of the face on which the spherical faces were formed and the opposing face of each of the KNbO 3 single crystal blocks on which the convex spherical faces had been formed in this manner. Prior to such coating, TSMR8900 photoresist made by Tokyo Ohka Kogyo Kabushiki Kaisha was first spin coated on the side of the KNbO 3 single crystal blocks on which the spherical faces were formed, and then pre-baking was performed to form a film of the thickness of 9 µm, whereafter the photoresist was left, by close contact exposure and developing processing, only at a window portion through which a waveform was to be admitted. In this condition, TiO 2 and SiO 2 were alternately electron beam vapor deposited by means of an electron beam vapor depositing apparatus of the model EX-550 made by Nippon Shinku Kabushiki Kaisha so as to vapor deposit them for 6 cycles in 12 layers in a condition wherein a maximum reflection factor was obtained at the wavelength of 860 nm and then for 12.5 cycles in 25 layers in another condition wherein a maximum reflection factor was obtained at the wavelength of 430 nm. The vapor deposition was performed intermittently so that the temperature of the specimens might be equal to or lower than 160 °C to the highest. The reflection factor of the coated films was 99.95 % at the wavelength of 860 nm and 99.9 % at the wavelength of 430 nm. After the specimens were taken out of the vapor depositing apparatus, they were immersed in acetone and washed using an ultrasonic washer to remove the photoresist and the coated films vapor deposited on the photoresist to form incidence windows. Subsequently, similar processing was applied also to the opposing faces, and coatings were provided and also higher harmonic wave output windows were formed. Each of them was set such that such construction as shown in Fig. 15 might be obtained, and laser light of the wavelength of 862 nm and the power of 78 mW was introduced into it such that the spherical faces having he radius of curvature of 10.12 mm might be used while keeping it at 31.7 °C. As a result, a higher harmonic output of the wavelength of 431 nm and the power of 1.2 mW was obtained.

Claims: An optical device constituted from a light transmitting optical material (40) having: a first face and a second face opposing each other; and a plurality of first reflecting portions (15) provided on said first face and a plurality of second reflecting portions (16) provided on said second face individually in an opposing relationship to said first reflecting portions (15), to provide a zigzag optical path between said first reflecting portions (15) and said second reflecting portions (16) alternately coupling said first reflecting portions (15) and said second reflecting portions (16) to each other; characterised in that at least one reflecting portion of a least one of said two pluralities of reflecting portions (15,16) is obtained by coating a reflecting film (13) on a convex arcuate face (12) on the appropriate first or second face. An optical device according to claim 1, wherein each of said first and second reflecting portions (15,16) comprises a coating of a reflecting film (13) on said optical material. An optical device according to claim 1 or 2, wherein each of said first reflecting portions (15) is convex arcuate and each of said second reflecting portions (16) is flat. An optical device according to claim 1 or 2, wherein each of said first and second reflecting portions (15,16) is convex arcuate. An optical device according to claim 1 or 2 wherein ones of said reflecting portions (16) are formed in prisms which reverse received light. An optical device according to any one of the preceding claims, wherein said light transmitting optical material (40) is a laser medium or non-linear optical crystal. An optical device according to claim 2 or any one of claims 3 to 6 when dependent on claim 2 wherein on each face said coating of reflecting film links each of the reflective portions (15,16) and has a window (17,18) in it. A laser apparatus comprising a laser resonator having an optical device according to any one of the preceding claims, wherein the light transmitting optical material is a laser medium. A short wavelength laser apparatus, comprising a higher harmonic wave converting device consisting of an optical device according to any one of claims 1 to 7, and a laser oscillator. A method of manufacturing an optical device as defined in claim 2 or 7, or any one of claims 3 to 6 when dependent on claim 2, said method including the steps of: forming at least one convex arcuate face (42a) in a photoresist film (42) formed on the surface of the light transmitting optical material (40) by exposure based on photolithography and development on said photoresist film (42); etching the surface of the light transmitting optical material (40) and the photoresist film (42a) to form, on the surface of the light transmitting optical material (40), said at least one convex arcuate face (12) similar to the convex arcuate face of the photoresist film (42a), and coating a reflecting film (13;14) on surfaces of the face on which the at least one convex arcuate face (12) is formed and another face opposing to the face on which the at least one convex arcuate face (12) is formed to provide said reflecting portions (15,16), wherein in said step of forming at least one convex arcuate face by exposure based on photolithography, when the photoresist film (42) is exposed to light of a circular or elliptic pattern, the intensity of exposure light is varied gradually or stepwise from the centre toward an outer periphery of the pattern. A method according to claim 10, wherein the exposing step uses a lens of a low resolution as an exposure lens to form an image of the circular or elliptic pattern of a photomask on the photoresist film to effect the exposure of the photoresist film (42). A method according to claim 10, wherein, in the exposing step, an image of the circular or elliptic pattern to be formed on the photoresist film is formed on the photoresist film (42) in a defocused condition to effect exposure of the photoresist film. A method according to claim 10, wherein, in the exposing step, when the photoresist film (42) is to be exposed to light using a photomask on which the pattern is formed, a black or black background circular or elliptic shape is photographed in a somewhat defocused condition to obtain a negative film having a circular or elliptic pattern which varies in photographic density from a central portion toward an outer periphery thereof, and the photoresist film (42) is exposed to light in a condition wherein an image of the negative film is formed on the photoresist film using the negative film as a negative for transfer of the circular or elliptic pattern, or the negative is disposed in the proximity of or in close contact with the photoresist film. A method according to claim 10, wherein, in the exposing step, an image of the pattern to be formed is formed on the photoresist film (42) using diffused light to effect exposure of the photoresist film. A method according to claim 14, wherein a diffuser is inserted intermediately of an optical system for exposure in order to form diffused light. A method according to claim 10, wherein, in the exposing step, the photoresist film (42) is exposed to light in a condition wherein the photomask on which the pattern is formed is spaced from the photoresist film so as to provide a distance between them. A method of manufacturing an optical device as defined in claim 2 or 7 or any one of claims 3 to 6 when dependent on claim 2, said method including the steps of: forming a pattern by photolithography on a photoresist film (42) which is formed on a surface of the light transmitting optical material (40) and has a generally flat and smooth upper end face, to form at least one circular column-shaped or elliptic column-shaped photoresist film; heat treating the photoresist film to deform the generally flat upper end face of the photoresist film into at least one convex arcuate face (42a); etching the surface of the light transmitting optical material (40) and the deformed photoresist film (42a) to form at least one convex arcuate face (12) similar to the at least one convex arcuate face of the photoresist film (42a) on the surface of the light transmitting optical material (40); and coating a reflecting film (13,14) on surfaces of the face of the light transmitting optical material (40) on which the at leat one convex arcuate face (12) is formed and another face of the light transmitting optical material (40) opposing the face on which the at least one convex arcuate face (12) is formed, to provide said reflecting portions (15,16). A method of manufacturing an optical device as defined in claim 7 according to any one of claims 10 to 17, wherein the step of providing the reflecting portions includes the steps of: spin coating photoresist (42) on the face of the light transmitting optical material (40) on which the at least one convex arcuate face is formed and the face opposing to the face on which the at least one convex arcuate face is formed and pre-baking the light transmitting optical material (40) to form a photoresist film (12), leaving, by close contact exposure and developing processing, the photoresist film (42) at an end portion of the first face and an opposite end portion of the first or second face at positions to correspond to said windows (17,18) in said reflecting film, and providing a coating on the face of the light transmitting optical material (40) on which the photoresist film remains and removing the photoresist film and the coated film formed on the photoresist film to form said windows (17,18). A method according to any one of claims 10 to 18, wherein the etching of the surface of the optical material (40) and the photoresist film (42) is performed by dry etching, and a condition of the dry etching is varied continuously or stepwise at any time during etching. A method according to any of claims 10 to 19 wherein, after mirror face polishing of a predetermined face of said optical material (40), the photoresist film (42) is formed on said predetermined face of the optical material (40), and then the photoresist film (42) and the polished face of the optical material (40) are dry etched.

Inventors: EDA AKIRA, MURO KIYOFUMI, EDA, AKIRA, MURO, KIYOFUMI, Eda, Akira, c/o Mitsui Petrochemical Ind., Ltd., Muro, Kiyofumi, c/o Mitsui Petrochemical Ind. Ltd.
Applicants: MITSUI CHEMICALS INC, MITSUI CHEMICALS, INC.
Application year: 1992
Main classification: G03F7
IPC codes: H01S5, G02B6, H01S3, G02B3, G03F7, G02F1