JP2013072883A - Optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical device which achieves high coupling efficiency without using an aspheric lens and at low cost.SOLUTION: An optical device 1 includes a light source 30, a ball lens 10 that condenses a light flux emitted from the light source 30, and a flat plate-like optical element 20 provided with an optical surface 20a for correcting the spherical aberration of the ball lens 10.

Description

  The present invention relates to an optical device that can obtain good coupling efficiency at low cost.

  Optical communication is widespread using an optical fiber as a transmission medium and a semiconductor laser as a light source. In optical communication, an electrical signal from a communication terminal is converted into an optical signal and transmitted, and an optical signal received via a transmission medium is restored to an electrical signal. An optical device integrated as an optical module is used as an input / output unit for transmission or relay or an input / output unit for reception.

  In such an optical device, an optical element such as a collimator lens or a condensing lens is used between a semiconductor laser as a light source and a light receiving unit to which an optical fiber is attached, and a beam from the semiconductor laser is coupled to the optical fiber with high coupling efficiency. . For the purpose of applying to such an optical device, for example, Patent Document 1 discloses a method for manufacturing a lens with a lens barrel. Patent Document 1 describes a manufacturing method in which a lens material is press-molded into an optimum shape as a condensing lens, and an optical functional film for reducing reflection loss in an optical element is provided.

  Generally, the size of the light emitting part of the semiconductor laser is about several μm, and the core diameter of the optical fiber as a transmission medium is about 10 μm. Therefore, in order to obtain good coupling efficiency, it is necessary to accurately align the light emitting unit, the light receiving unit to which the optical fiber is attached, and the necessary optical elements in assembling the optical device. Therefore, in order to facilitate the assembly of the optical device, an optical module in which a spherical lens (ball lens) is arranged on a substrate as a collimating lens and a condensing lens as disclosed in Patent Document 2 is disclosed.

Japanese Patent Laid-Open No. 9-258084 JP 2002-341189 A JP 2006-525889 A

  However, if the divergence angle from the light source is large, the spot diameter of the condensing point becomes large due to the spherical aberration of the ball lens, and the coupling efficiency to the optical fiber decreases. Therefore, in an optical apparatus using a light source with a large divergence angle, it is necessary to correct spherical aberration by using an aspherical lens whose optical surface is rotationally symmetric about the optical axis as a collimating lens or a condensing lens.

  In order to manufacture an aspheric lens, it is necessary to heat and soften the glass base material and press-mold it using a mold to make the lens surface an aspherical shape. For this reason, it is necessary to precisely process the transfer surface of the mold into a complicated aspherical shape, and there is a problem that the manufacturing cost is high.

  Thus, Patent Document 3 discloses a method for manufacturing an aspheric hybrid lens in which a polymer resin for an aspheric lens is integrated with a spherical lens (ball lens). However, when a polymer resin is used for an aspherical lens, there is a problem that a focus shift due to deformation or moisture absorption occurs in an environment of high temperature and high humidity. For this reason, even with an aspherical hybrid lens, if optical performance and stability such as high-precision aberration and focal position are required, the optical glass must be press-molded, so the production cost is high. It did not change.

  Therefore, in order to correct spherical aberration and obtain a good coupling efficiency, an optical glass aspherical lens must be used, and high-precision and stable optics can be used over a long period of time even in high-temperature and high-humidity environments. The device was expensive.

  The present invention has been made to solve the above-described problems, and in particular, an object of the present invention is to provide an optical device that can obtain good coupling efficiency at low cost.

  The optical device of the present invention includes a light source, a ball lens that collects a light beam emitted from the light source, and a flat plate-like optical element provided with an optical surface that corrects spherical aberration of the ball lens. It is characterized by that.

  Accordingly, an optical device in which spherical aberration is corrected can be realized at low cost using a ball lens and a flat plate-shaped optical element provided with an optical surface for correcting spherical aberration. Since the spherical aberration is corrected, the coupling efficiency to an optical fiber or the like can be improved.

  Therefore, good coupling efficiency can be obtained at low cost.

  Furthermore, the optical element is preferably a diffractive optical element provided with a diffraction grating surface. By forming a diffraction grating surface on one surface of a flat plate, it is possible to correct spherical aberration with high accuracy at low cost, and because it does not use polymer resin, it is stable in high temperature and high humidity environments. It can be a device.

  It is preferable that the optical element and the ball lens face each other with a gap. In this way, position adjustment in the assembly of the optical device is possible, and the mass productivity is excellent.

  It is preferable that a lens barrel for holding the ball lens and the optical element is provided, the ball lens is fixed in the lens barrel, and the optical element is airtightly fixed to the lens barrel. In this way, an optical device having a hermetically sealed structure can be obtained by using a lens with a lens barrel, so that deterioration of the semiconductor laser can be prevented over a long period of time.

  An optical device according to the present invention includes a light source, a first ball lens that collimates a light beam emitted from the light source, and a flat plate-like first optical element that corrects spherical aberration of the first ball lens. A second ball lens that collects the parallel light transmitted through the first optical element, and the second ball lens disposed between the first optical element and the second ball lens. And a flat plate-like second optical element that corrects the spherical aberration.

  Accordingly, an optical device in which spherical aberration is corrected can be realized at low cost by using a ball lens and a flat plate-like optical element that corrects spherical aberration. In addition, since the spherical aberration is corrected, the coupling efficiency to an optical fiber or the like can be improved. Furthermore, since the light beam that has passed through the first ball lens is parallel light, it is possible to easily insert a beam splitter or an optical isolator into the optical device.

  According to the optical apparatus of the present invention, since spherical aberration is corrected using a ball lens and a flat plate-shaped optical element provided with an optical surface for correcting spherical aberration, good coupling efficiency is obtained at low cost. be able to.

It is sectional drawing which shows the optical apparatus of 1st Embodiment. It is a schematic cross section of the flat optical element in the optical apparatus of 1st Embodiment. It is sectional drawing explaining the spherical aberration of a ball lens, (a) is a schematic diagram which shows the focal position which has not corrected spherical aberration, and (b) is the focal position which corrected spherical aberration. It is a schematic cross section which shows the modification of the flat type optical element in the optical apparatus of 1st Embodiment. It is sectional drawing which shows the optical apparatus of 2nd Embodiment.

<First Embodiment>
A first embodiment of the present invention will be described with reference to the drawings. Note that the dimensions of the drawings are changed as appropriate for easy understanding. FIG. 1 is a cross-sectional view showing the optical device 1 of the first embodiment, and FIG. 2 is a schematic cross-sectional view of a flat optical element 20 in the optical device 1. As shown in these drawings, the optical device 1 in the present embodiment includes a ball lens 10 and a flat optical element 20. Here, the “flat plate” in this specification is, for example, a shape having an optical surface processed from a flat plate surface, and is a state in which a “shape substantially close to a flat plate” is maintained as a whole.

  The ball lens 10 is a lens obtained by processing optical glass into a spherical shape. The flat optical element 20 is an optical glass plate having a diffraction grating surface 20a as shown in FIG. The light source 30 is a semiconductor laser that transmits an optical signal for optical communication. As shown in FIG. 1, in this embodiment, a ball lens 10 and a plate-like optical element 20 are fixed to a lens barrel 40 processed from stainless steel, and a substrate (not shown) that holds a light source 30 and a lens barrel 40. The formed flange portion 40a is welded to form the optical device 1. An optical fiber end face (not shown) is connected to the optical device 1, and the beam from the light source 30 is focused toward the light receiving unit 60 corresponding to the end face of the optical fiber.

  Since the ball lens 10 is equivalent in any direction, there is no concern about an angular deviation or the like if the center of the lens coincides with the optical axis. Moreover, it is relatively inexpensive as a lens. For this reason, it is used as a condensing lens on the side of a light receiving element such as a photodiode having a large light receiving area. Also, it is often used as a condenser lens that couples collimated light to an optical fiber. However, since the ball lens 10 is spherical, spherical aberration cannot be ignored.

  FIG. 2 is a schematic view showing a cross section of the flat optical element 20. A rotationally symmetric diffraction groove is processed on the optical axis on the surface facing the ball lens 10, and this surface is a diffraction grating surface 20 a that corrects the spherical aberration of the ball lens 10. The other surface of the optical element 20 is a flat processed surface 20b. The diffraction grating surface 20a can be mass-produced by a microfabrication technique used in LSI (Large-Scale Integration) or MEMS (Microelectromechanical system), and therefore, the cost can be reduced compared to press molding of a glass lens. is there. In addition, by forming a diffraction grating surface 20a on one surface of an optical glass plate, low-cost and highly accurate spherical aberration correction can be performed, so even in a high temperature and high humidity environment. Stable optical characteristics can be obtained.

  The diffraction grating surface 20a has a concentric grating periodic structure, and the grating period (grating pitch) becomes smaller toward the periphery. Moreover, the grating | lattice shape is a sawtooth shape of the grating | lattice pitch along a phase function. The thickness of the grating portion is several μm, and the grating pitch is gradually changed with a width of several hundred μm to several μm. In the diffraction grating, the smaller the grating pitch, the larger the amount of diffraction (diffraction angle), and the light traveling direction can be controlled by changing the grating pitch. Therefore, by forming a concentric grating periodic structure with a grating pitch calculated from a phase function, the same effect as an aspheric lens can be obtained, and spherical aberration can be corrected.

  3A and 3B are cross-sectional views for explaining spherical aberration of the ball lens. FIG. 3A is a schematic diagram showing a focal position where the spherical aberration is not corrected, and FIG. 3B is a schematic diagram showing a focal position where the spherical aberration is corrected. FIG. 3A shows a state where the focal positions are not matched due to the spherical aberration of the ball lens 10. When the beam diffused from the point light source is collected by the ball lens 10, the light that has passed through a position relatively away from the optical axis is collected at a relatively short focal position (P1) and relatively on the optical axis. The light passing through the near position is condensed at a relatively far focal position (P2), and spherical aberration is generated in which the focal position is distributed.

  For this reason, when the end face of the optical fiber is positioned at the condensing point, the spot diameter is expanded at any position of P1 and P2. Since the core diameter of the optical fiber is about 10 μm, there is a problem in that the coupling efficiency decreases when the spot diameter increases.

  As shown in FIG. 3B, when a flat optical element 20 provided with a diffraction grating surface 20a for correcting the spherical aberration of the ball lens 10 is disposed so as to face the ball lens 10, relatively light is emitted. Spherical aberration so that the light passing through the position away from the axis moves further in the focal position (P1 ′) and substantially coincides with the focal position (P2 ′) of the light passing through the position relatively close to the optical axis. Can be corrected.

  As described above, since the diffractive optical element 20 having the diffraction grating surface 20a corrects the spherical aberration with respect to the beam emitted from the ball lens 10, the beam emitted from the optical element 20 has almost spherical aberration. Without being condensed. Therefore, an optical device in which spherical aberration is corrected can be obtained by using the low-cost ball lens 10 and the optical element 20 provided with the diffraction grating surface 20a. By doing so, the spot diameter does not expand, so that the coupling efficiency to the optical fiber can be improved.

  Therefore, good coupling efficiency can be obtained at low cost.

  When the divergence angle of the beam from the light source 30 is large, the spread of the beam incident on the ball lens 10 is increased, so that the influence of spherical aberration is increased. In the present embodiment, an optical device in which spherical aberration is corrected by correcting aberration using the optical element 20 provided with the diffraction grating surface 20a is effective for the light source 30 having a large divergence angle. It is.

  The diffractive optical element 20 used in this embodiment can be manufactured as follows.

  As an optical glass flat plate used as a material, a glass flat plate having a thickness of 0.5 mm having a trade name of D263T manufactured by Schott Co., Ltd. was used, and a size allowing fine processing, for example, a diameter of 150 mm was used. A resist mask having a predetermined pattern was prepared by a photolithography method using a photosensitive resist on one surface of the optical glass flat plate. Next, a diffraction groove was formed by etching by RIE (Reactive Ion Etching) method. If necessary, the photolithography method and the etching process are repeated to form the diffraction grating surface 20a, and then a protective film is applied and cut into a size (for example, 2 mm square) to be applied to the optical device 1, and then the protective film is removed. After removing and washing, a flat optical element 20 was obtained. Since this method is superior in mass productivity as compared with the case of processing optical elements one by one, the cost can be reduced. In addition, since an optical glass plate is used, high-precision spherical aberration correction is possible.

  The optical device 1 was manufactured as follows.

  The ball lens 10 was attached to a lens barrel 40 processed with stainless steel by sealing with low-melting glass, and an antireflection film was deposited on both surfaces of the ball lens 10 using a vacuum deposition apparatus. Next, the optical element 20 was hermetically attached and fixed to the lens barrel 40 by sealing with a low melting point glass so that the ball lens 10 and the diffraction grating surface 20a face each other with a predetermined gap. The optical element 20 is preferably formed with an antireflection film in advance as necessary. Further, instead of sealing with low-melting glass, adhesion with UV resin or the like can be used.

  Finally, the substrate to which the light source 30 was attached was joined to the flange portion 40a of the lens barrel 40 by welding to constitute the optical device 1.

  As an optical module having an emission wavelength of the light source 30 of 1310 nm, the coupling efficiency to the optical fiber was examined. The ball lens 10 was a product name LASFN9 manufactured by Schott and having a diameter of 1.8 mm. The refractive index at this wavelength was 1.8174. In addition, a glass flat plate having a thickness of 0.5 mm made by Schott product name D263T was used as the optical element 20, and a diffraction grating surface 20a was formed on one surface. The light emitting point of the light source 30 and the gap between the ball lens 10 are arranged to be 0.5 mm, and the gap between the ball lens 10 and the optical element 20 is 0.2 mm. At this time, the thickness of the grating portion of the diffraction grating surface 20a (depth from the upper surface to the deepest processing surface) is 2.57 μm, and the grating pitch (the first deepest surface from the optical axis, or From the deepest surface to the next deepest surface) were formed in the radial direction from the optical axis in a width of 249 μm, 31.8 μm, 21.4 μm, 14.5 μm, 10.4 μm, and 7.9 μm. As shown in FIG. 2, the diffraction grooves in this embodiment have a stepped shape of 8 steps per period. At this time, the gap between the optical element 20 and the optical fiber was adjusted to 2.207 mm. A coupling efficiency of 97.23% was obtained when a semiconductor laser with an NA of 0.22 and an optical fiber of NA of 0.09 was attached. On the other hand, for comparison, the coupling efficiency when the optical element 20 is not inserted is 66.44% at the maximum, and it was confirmed that the coupling efficiency can be greatly improved by correcting the spherical aberration.

  Similarly, the ball lens 10 was manufactured using a product name K-PSFN202 manufactured by Sumita Optical Glass Co., Ltd. having a diameter of 2.0 mm. In this case, the refractive index was 1.9654. The light emitting point of the light source 30 and the gap between the ball lens 10 are arranged to be 0.4216 mm, and the gap between the ball lens 10 and the optical element 20 is 0.2 mm. At this time, the thickness of the grating portion of the diffraction grating surface 20a is 2.57 μm, and the grating pitch is 245 μm, 42.7 μm, 24.7 μm, 17. It was formed to have a width of 2 μm and 13.1 μm. The diffraction groove has a stepped shape of 8 steps per period. At this time, the gap between the optical element 20 and the optical fiber was adjusted to 2.186 mm. When a light source 30 having a semiconductor laser NA of 0.22 and an optical fiber NA of 0.09 was attached, a coupling efficiency of 98.35% was obtained. On the other hand, the coupling efficiency when the optical element 20 was not inserted was 73.15% at the maximum.

  In addition, although the optical fiber was used in order to evaluate the coupling efficiency in the case of applying to optical communication, this embodiment is not limited to an optical fiber.

  In the present embodiment, the flat optical element 20 and the ball lens 10 are opposed to each other with a gap. By doing so, the position adjustment in the assembly of the optical device 1 can be performed, and fine adjustment can be made to the dimensional variation of the ball lens diameter, which is excellent in mass productivity.

  Moreover, since the board | substrate which attached the light source 30 is joined to the collar part 40a of the lens-barrel 40 by welding, the light source 30 can be made into an airtight sealing structure. Practically, an inert gas such as argon gas is sealed when welding. In this way, deterioration due to oxidation can be prevented over a long period of time.

  It is preferable that a diffraction grating surface 20a for correcting spherical aberration is disposed so as to face the ball lens 10. In this way, an optical design of the diffraction grating surface 20a suitable for correcting the aberration of the ball lens 10 is facilitated without any medium other than air. The other surface of the optical element 20 is preferably a flat plate processed surface. Since an optical flat plate may be used, the cost is low and the mass productivity in the manufacturing process is excellent. For the optical axis alignment when attaching to the lens barrel 40, a method using the side surface of the optical element 20 as a reference surface or a method of providing an alignment mark on the optical element 20 can be employed.

  FIG. 4 shows a modification of the optical element 20. The diffraction grating surface 20a shown in FIG. 2 is a diffractive type (phase type), but the optical element 20 shown in FIG. 4 is provided with a refractive lens surface 20c. A refractive lens surface 20c as an optical surface for correcting spherical aberration has a concentric geometric shape similar to that of a Fresnel lens. Compared with the step-like diffraction grooves shown in FIG. 2, it is difficult to form the lens surface 20c shown in FIG. 4 by photolithography and etching, but the spherical aberration correction as the flat optical element 20 is equivalent. .

<Second Embodiment>
FIG. 5 is a cross-sectional view showing the optical device 2 of the second embodiment. In this embodiment, in addition to the ball lens 11 disposed on the light source 31 side, the ball lens 12 disposed on the optical fiber 71 side and the ball lens 15 disposed on the light receiving element 50 side such as a photodiode are provided. Have. Further, the optical device 2 is configured to include a flat plate-like first optical element 21 that corrects the spherical aberration of the ball lens 11 and a flat plate-like second optical element 22 that corrects the spherical aberration of the ball lens 12. ing.

  Here, the ball lens 11 is arranged as a collimating lens. The light beam emitted from the light source 31 enters the ball lens 11 and is emitted toward the optical element 21 as parallel light. The optical element 21 has a diffraction grating surface 21 a formed on the surface facing the ball lens 11 to correct the spherical aberration of the ball lens 11.

  On the other hand, the ball lens 12 arranged on the optical fiber 71 side condenses the parallel light transmitted through the optical element 22 on the end face of the optical fiber 71. At this time, the spherical aberration of the ball lens 12 is corrected by the diffraction grating surface 22 a provided on the optical element 22. Thereby, since it can suppress that the spot diameter of the light beam condensed on the end surface of the optical fiber 71 expands due to spherical aberration, the coupling efficiency can be improved.

  Further, the optical device 2 can couple the light beam emitted from the optical fiber 71 to the light receiving element 50. By combining differently the wavelength of the light beam emitted from the optical fiber 71 and the wavelength of the light source 31, a single-core bidirectional optical module that performs transmission and reception with one optical fiber 71 and one optical device 2 is realized. it can.

  In the present embodiment, parallel light is obtained between the flat first optical element 21 and the second optical element 22. An isolator 51 and a beam splitter 52 are inserted so that a light beam emitted from the optical fiber 71 enters the ball lens 15 and prevents return light to the light source 31. The light beam that has passed through the ball lens 15 is condensed on the light receiving element 50 via the wavelength selection filter 53. Since the light receiving area of the light receiving element 50 is relatively large, it is not affected by the spread of the spot diameter due to the spherical aberration of the ball lens 15.

  Since the light beam that has passed through the ball lens 11 is parallel light, the occurrence of aberration due to the insertion of the isolator 51 and the beam splitter 52 can be suppressed. Therefore, even if the isolator 51 and the beam splitter 52 are inserted, the coupling efficiency can be improved.

  As an optical module having an emission wavelength of the light source 31 of 1310 nm, the coupling efficiency to the optical fiber was examined. The ball lens 11 was a product name LASFN9 manufactured by Schott and having a diameter of 1.8 mm. The refractive index at this wavelength was 1.8174. Further, as the optical element 21, a glass plate having a thickness of 0.5 mm with a trade name of D263T manufactured by Schott was used, and a diffraction grating surface 21a was formed on one surface. The light emitting point of the light source 31 and the gap between the ball lens 11 are arranged to be 0.1005 mm, and the gap between the ball lens 11 and the optical element 21 is 0.2 mm. At this time, the thickness of the grating portion of the diffraction grating surface 21a is 2.57 μm, and the grating pitch is 359 μm, 62.5 μm, 51.6 μm, 50. The width was 6 μm. The diffraction groove has a stepped shape of 8 steps per period.

  As the ball lens 12 disposed on the optical fiber 71 side, a product name LASFN 9 having a diameter of 4.5 mm was used. Like the optical element 21, the optical element 22 has a diffraction grating surface 21a on one side, the thickness of the grating portion is 2.57 μm, and the grating pitch is a diameter from the optical axis so as to correct the spherical aberration of the ball lens 12. In the direction, the widths of 534 μm, 150 μm, and 26.8 μm were formed. The diffraction groove has a stepped shape of 8 steps per period. The interval between the optical element 21 and the optical element 22 may be arbitrary, but is 2 mm.

  When the isolator 51 and the beam splitter 52 are not inserted, and the NA of the semiconductor laser of the light source 31 is 0.22 and the NA of the optical fiber 71 is 0.09, the coupling efficiency at the optimum position is used. 99.49% was obtained. On the other hand, for comparison, the coupling efficiency when the optical element 21 and the optical element 22 were not inserted was 93.83% at the maximum. Thus, it was confirmed that the coupling efficiency can be improved by correcting the spherical aberration.

  Therefore, an optical device in which spherical aberration is corrected can be obtained by using the low-cost ball lenses 11 and 12 and the optical elements 21 and 22 provided with the diffraction grating surfaces 21a and 22a. Since the spherical aberration is corrected, the coupling efficiency to the optical fiber 71 and the like can be improved. Furthermore, since the light beam that has passed through the ball lens 11 is parallel light, the isolator 51 and the beam splitter 52 can be easily inserted.

  Also in this embodiment, it is needless to say that the condensing point is not limited to the end face of the optical fiber 71.

  In the first embodiment and the second embodiment, the diffraction grating surface is formed on one surface of the optical glass flat plate, but the other surface may be processed into a surface having a lens action. However, in consideration of the accompanying increase in cost, it is preferable to form a diffraction grating surface on the other surface.

1, 2, Optical device 10, 11, 12, 15 Ball lens 20, 21, 22 Optical element 20a, 21a, 22a Optical surface (diffraction grating surface)
20b Flat processing surface 20c Optical surface (lens surface)
30, 31 Light source (semiconductor laser)
40 Lens tube 40a collar 50 Light-receiving element (photodiode)
51 Isolator 52 Beam Splitter 53 Wavelength Selection Filter 60 Light Receiving Unit 71 Optical Fiber

Claims (5)

  An optical apparatus comprising: a light source; a ball lens that condenses a light beam emitted from the light source; and a flat plate-like optical element provided with an optical surface that corrects spherical aberration of the ball lens.   The optical apparatus according to claim 1, wherein the optical element is a diffractive optical element provided with a diffraction grating surface.   The optical device according to claim 2, wherein the optical element and the ball lens face each other with a gap.   2. A lens barrel for holding the ball lens and the optical element is provided, the ball lens is fixed in the lens barrel, and the optical element is airtightly fixed to the lens barrel. The optical device according to claim 3.   A light source; a first ball lens that collimates a light beam emitted from the light source; a flat plate-like first optical element that corrects spherical aberration of the first ball lens; and the first optical element. A second ball lens that collects the parallel light that has passed through the plate, and a flat plate that is disposed between the first optical element and the second ball lens and corrects spherical aberration of the second ball lens. And an optical second device.