Optical device

Abstract

This invention discloses an optical device comprising a first lens and a second lens. The first lens of the optical device is coupled to the end face of the optical waveguide of the optical element to radiate the outgoing light from the optical element. The second lens is optically coupled to the first lens to convert the outgoing light from the first lens into collimated light.

Description

[0001] This application is a divisional application of Chinese National Application No. 202111209206.1 (Optical Devices) filed on October 18, 2021, the contents of which are quoted below. Technical Field

[0002] This invention relates to optical devices. This application claims priority to Japanese Application No. 2020-178441, filed on October 23, 2020, the entire contents of which are incorporated herein by reference. Background Technology

[0003] Japanese Patent Application Publication No. 2001-281501 discloses a laser diode module that optically couples a laser diode and an optical fiber. In this module, in order to guide the emitted light from the laser diode into the optical fiber, the laser diode and the incident end face of the optical fiber are fixed in a predetermined positional relationship, and a focusing lens for optical coupling with the laser diode and the optical fiber is provided between them. Summary of the Invention

[0004] One aspect of the present invention relates to an optical device comprising: a first lens which is coupled to the end face of the optical waveguide of an optical element to radiate outgoing light from the optical element; and a second lens which is optically coupled to the first lens to convert outgoing light from the first lens into collimated light.

[0005] Another aspect of the present invention relates to an optical device comprising: a first lens which is coupled to the end face of the optical waveguide of the optical element to focus incident light toward the optical element; and a second lens which is optically coupled to the first lens to convert collimated light into incident light toward the first lens. Attached Figure Description

[0006] Figure 1 This is a diagram illustrating an example of an optical device according to one embodiment of the present invention.

[0007] Figure 2 yes Figure 1 An oblique view of the optical device shown.

[0008] Figure 3 It is used for explanation Figure 1 The diagram shows the spread of light from the optical waveguide in the optical device.

[0009] Figure 4 It means Figure 1 The diagram shows the optical path of the optical device from the optical element to the optical fiber.

[0010] Figure 5 Is Figure 1The diagram shows the optical device with the optical axis of the optical waveguide and the optical axis of the first lens misaligned.

[0011] Figure 6 It means Figure 1 The diagram shows the relationship between the increase in loss and the reflection return rate of the optical device and the axial misalignment of the optical waveguide and the first lens.

[0012] Figure 7 It means in Figure 1 The diagram shows the characteristics of the optical device, where the magnification of the first lens is changed, and the loss caused by vignetting is related to the beam diameter of the optical waveguide.

[0013] Figure 8 It means in Figure 1 The diagram shows the characteristics of the optical device, with the lens thickness set to a constant, and the relationship between the magnification and the distance from the front end of the lens to the virtual image relative to the radius of curvature of the hemispherical portion.

[0014] Figure 9 It means Figure 1 The diagram shows the relationship between the loss caused by aberrations in the optical device and the radius of curvature of the lens.

[0015] Figure 10 This is a diagram used to illustrate the relationship between the aberrations and virtual image of the lens when the thickness of the first lens is increased.

[0016] Figure 11 It is used to explain in Figure 1 The optical device shown is an aspherical lens used in the first lens.

[0017] Figure 12A This is a diagram showing another example of an aspherical lens.

[0018] Figure 12B This is a diagram showing other examples of aspherical lenses.

[0019] Figure 12C This is a diagram showing other examples of aspherical lenses.

[0020] Figure 13 It is a diagram showing the optical path of an existing optical device from the optical element to the optical fiber. Detailed Implementation

[0021] [The problem to be solved by the present invention]

[0022] The laser diode module disclosed in Japanese Patent Application Publication No. 2001-281501 requires extremely high precision positioning of the laser diode, focusing lens, and optical fiber components in order to efficiently guide light from the laser diode to an external optical fiber. Similarly, optical circuits that use optical waveguides formed on a substrate to densely integrate many optical elements also require efficient input of light from the optical waveguide of the optical circuit to external components, and efficient guidance of light from external components to the optical waveguide of the optical circuit. Therefore, interfaces with good optical coupling efficiency to external components are required.

[0023] [Effects of the Invention]

[0024] According to the present invention, an optical device with good optical coupling efficiency with the outside can be obtained.

[0025] [Description of embodiments of the present invention]

[0026] First, embodiments of the present invention will be described. One embodiment of the optical device includes: a first lens, which is coupled to the end face of the optical waveguide of an optical element to radiate light emitted from the optical element; and a second lens, which is optically coupled to the first lens to convert the light emitted from the first lens into collimated light. Thus, an optical device with good optical coupling efficiency to the outside can be obtained.

[0027] One embodiment of the optical device according to the present invention may further include: a third lens that focuses the collimated light emitted from the second lens; and an optical fiber that is optically coupled to the third lens for receiving the focused light emitted from the third lens. Thus, light from the optical device can be efficiently guided into the optical fiber.

[0028] In one embodiment of the present invention, the optical element in the optical device can be any of a laser element, an optical modulation element, or an optical amplification element. Therefore, an optical device having an optical element that emits light to the outside can also be obtained with good optical coupling efficiency with the outside.

[0029] One embodiment of the present invention relates to an optical device comprising: a first lens, which is coupled to the end face of the optical waveguide of an optical element to focus incident light onto the optical element; and a second lens, which is optically coupled to the first lens to convert collimated light into incident light toward the first lens. Thus, an optical device with good optical coupling efficiency to the outside can be obtained.

[0030] One embodiment of the optical device according to the present invention further includes: a third lens that directs the collimated light into the second lens; and an optical fiber that is optically coupled to the third lens and emits incident light directed into the third lens. Thus, light from the optical fiber can be efficiently guided into the optical device.

[0031] In one embodiment of the present invention, the optical element in the optical device can be any of an optical modulation element, a waveguide-type light-receiving element, or an optical amplification element. Therefore, an optical device having an optical element that receives light from the outside can also be obtained with good optical coupling efficiency to the outside.

[0032] In one embodiment of the optical device according to the present invention, the first lens may be an aspherical lens having a curved surface convex to the opposite side relative to the optical element. In the first lens, the region that engages with the end face of the optical waveguide of the optical element can be formed as a plane orthogonal to the optical axis of the first lens. The area of ​​this plane may be smaller than the maximum area of ​​a cross-section orthogonal to the optical axis of the optical waveguide in the first lens. This allows for the suppression of light loss caused by aberrations in the first lens.

[0033] In one embodiment of the present invention, the optical device can be such that, when the first lens is set to a refractive index n, a predetermined position on the surface of the optical element side is designated as point O, any position on the surface of the first lens other than the surface of the optical element side is designated as point S, and the position of the virtual image formed by the first lens is designated as point P, the value obtained by subtracting the distance between points S and P from the value obtained by multiplying the distance between points O and S by the refractive index n is a constant value. Therefore, light loss caused by aberrations of the first lens can be suppressed.

[0034] In one embodiment of the present invention, the optical device may have its optical axis orthogonal to the plane on the side of the optical element at the position of point O of the first lens, and the position point P of the virtual image of the first lens is located on the optical axis of the optical waveguide. Therefore, the first lens can be formed symmetrically with respect to the optical axis, thus simplifying manufacturing.

[0035] In one embodiment of the present invention, the optical device may have its optical axis orthogonal to the surface of the optical element at point O of the first lens, and the virtual image P of the first lens is located at a position separated from the optical axis of the optical waveguide. This suppresses light loss caused by aberrations of the first lens and reduces the impact of reflected light from the surface of the first lens.

[0036] In one embodiment of the optical device according to the present invention, the optical axis of the optical waveguide can be orthogonal to the surface of the optical element at a position other than point O. This allows for the suppression of light loss caused by aberrations of the first lens and reduces the impact of reflected light from the surface of the first lens.

[0037] In one embodiment of the optical device according to the present invention, the first lens may have at least a hemispherical portion on the side optically coupled to the second lens. This suppresses the propagation of light emitted from the optical waveguide and reduces the optical power of the second lens. Furthermore, it increases the allowable alignment accuracy during the installation of each component.

[0038] In one embodiment of the optical device according to the present invention, the first lens may be a lens with a thickness greater than the radius of curvature of the hemispherical portion in the optical axis direction. Therefore, even with a larger magnification of the first lens and a smaller beam diameter of the optical waveguide, light propagation can be suppressed.

[0039] In one embodiment of the present invention, the optical device may have its optical axis offset relative to the optical waveguide of the optical element by more than half the beam diameter of the optical waveguide. This reduces the impact of reflected light from the surface of the first lens.

[0040] In one embodiment of the optical device according to the present invention, the first lens can be made of either glass or resin. This increases the options for lens materials. Furthermore, when using resin, it can be formed using 3D printing technology.

[0041] [Detailed description of the embodiments of the present invention]

[0042] Hereinafter, a specific example of an optical device according to an embodiment of the present invention will be described with reference to the accompanying drawings. The present invention is not limited to the following examples, but is shown in the claims and includes all modifications within the meaning and scope equivalent to the claims. Furthermore, the present invention includes combinations of any of the embodiments, as long as multiple embodiments can be combined. In addition, in the following description, components that are labeled with the same reference numerals in different drawings are the same components, and their description is sometimes omitted.

[0043] [First Embodiment]

[0044] In this embodiment, a laser element is used as an example of an optical element used in optical devices. However, it is not limited to a laser element as long as it is an optical element that emits light to the outside from an optical waveguide. It can also be an optical modulation element or a brightness enhancement element. Figure 1This is a diagram illustrating an example of an optical device according to one embodiment of the present invention. Figure 2 yes Figure 1 An oblique view of the optical device shown. Figure 3 It is used for explanation Figure 1 The diagram shows the spread of light from the optical waveguide in the optical device. Figure 4 It means Figure 1 The diagram shows the optical path of the optical device from the optical element to the optical fiber. Furthermore, in... Figure 4 Descriptions of optical components and optical fibers have been omitted.

[0045] The optical device 1 according to this embodiment includes, as a component of an optical system, a laser element, namely a laser diode chip (hereinafter referred to as "LD chip") 40, a first lens 10, a second lens 20, a third lens 30, and a single-mode fiber (SMF), namely an optical fiber 60. The LD chip 40 is mounted on a sub-carrier 52 made of a material with high thermal conductivity, such as aluminum nitride. The second lens 20 is a collimating lens that converts light incident from the first lens into a parallel beam, i.e., collimated light, as described later, and is fixed to a carrier 51 together with the sub-carrier 52 on which the LD chip 40 is mounted. The carrier 51 is disposed, for example, in a package 50 using an Fe-Ni-Co alloy (e.g., trade name Kova iron-nickel-cobalt alloy). The drive signal for the LD chip 40 is supplied from outside the package 50 via a feedthrough 53.

[0046] The first lens 10, formed of resin, is bonded to the end face of the optical waveguide 41 of the LD chip 40. When the first lens 10 is formed of resin, it can be fabricated using 3D printing technology. The first lens 10 and the second lens 20 are optically coupled, and the second lens, in a centered state, is precisely fixed to the carrier 51, for example, using an adhesive such as epoxy resin. The optical fiber 60 is positioned and held within the socket 62, housed within the ferrule 61. The third lens 30 is a condenser lens. Collimated light from the second lens 20 is focused by the third lens 30 and coupled to the optical fiber 60. The third lens 30 is housed in a retainer 63 disposed outside the package 50. The third lens 30 and the optical fiber 60 are centered to achieve optical coupling. The retainer 63 and the package 50 are fixed, for example, by YAG bonding.

[0047] The LD chip 40 is an optical element with an optical waveguide 41. The LD chip 40 can be, for example, a semiconductor laser with an optical waveguide or a semiconductor laser with a double heterojunction. In a semiconductor laser with a double heterojunction, the active layer has a larger refractive index than the surrounding cladding to form an optical waveguide, thus becoming an optical element with an optical waveguide. Figure 3As shown, in this embodiment, the first lens 10 is formed as a hemispherical lens and is disposed in conjunction with the end face of the optical waveguide 41 of the LD chip 40. The optical axis (central axis) of the first lens 10 is disposed without offset in a manner consistent with the optical axis of the optical waveguide 41 of the LD chip 40.

[0048] The first lens 10 of the optical device 1 is disposed on the end face of the optical waveguide 41 of the LD chip 40. Therefore, as Figure 4 As shown, the light emitted from the optical waveguide 41 of the LD chip 40 (not shown) is efficiently guided by the first lens 10 to the second lens 20. After being transformed into collimated light A by the second lens 20, the light emitted from the LD chip 40 is focused by the third lens and guided to the optical fiber 60 (not shown). Therefore, compared with the past, the optical device 1 achieves a high optical coupling efficiency with the external optical fiber 60.

[0049] Here, the differences between the optical device according to one embodiment of the present invention and existing optical devices will be explained. Figure 13 This is a diagram representing the optical path of an existing optical device from the optical element to the optical fiber, used as a model for comparison. Figure 4 Similarly, the description of optical elements and optical fibers is omitted. In the existing optical device 101, there is also a first lens 110, a collimating lens (i.e., a second lens 120), and a condensing lens (i.e., a third lens 30). The light emitted from the optical waveguide 41 of the LD chip 40 (not shown) is guided by the first lens 110 to the second lens 120, and after being transformed into collimated light A by the second lens 120, it is focused by the third lens 30 and guided to the optical fiber 60 (not shown).

[0050] Figure 13 The optical device 101 shown illustrates the case where a Gaussian beam with a beam radius ω of 0.8 μm and a wavelength λ of 1.55 μm is incident on a first lens 110 with a numerical aperture NA of 0.65 onto an optical waveguide. Here, the numerical aperture NA of the optical waveguide 41 is calculated using NA = λ / (π × ω), and is 0.62. As per... Figure 13 As seen in the magnified view, due to the diffusion of light from the optical waveguide, peripheral vignetting occurs in the first lens 110, as shown by dashed line B. Figure 13 In the case of the optical device 101 shown, the light loss is 1.0 dB. Furthermore, typically, if the numerical aperture NA of the beam is 0.6 or greater (beam radius ω is 0.8 μm or less), the light spread from the optical waveguide increases, making it difficult for the lens to receive the light. Therefore, light loss occurs.

[0051] Next, the optical device 1 involved in this embodiment will be described. Figure 3 This is a diagram illustrating the spread of light from optical waveguide 41 in the optical device of this embodiment. Figure 3 The optical device 1 shown is similar to the existing optical device 101 (see reference). Figure 13 Similarly, the optical waveguide 41 has a numerical aperture NA of 0.62, from which a Gaussian beam with a beam radius ω of 0.8 μm and a wavelength λ of 1.55 μm is emitted. The first lens 10 is a hemispherical lens with a refractive index n of 1.5 and a radius of curvature R of 50 μm. The second lens 20 is a collimating lens with a numerical aperture NA of 0.65.

[0052] like Figure 3 As shown, in this embodiment, the optical device 1 has a first lens 10 formed in a manner that is joined to the end face of the optical waveguide 41 of the LD chip 40. Since there is no air between the optical device 1 and the LD chip 40, light from the optical waveguide 41 does not leak and enters the first lens 10. When the refractive index of the first lens 10 is set to refractive index n, the propagation of light from the LD chip 40 becomes 1 / n times that in air due to refraction when it enters the first lens 10. On the other hand, light entering from the center of the hemispherical lens, i.e., the first lens 10, is perpendicular to the lens surface and does not refract upon exiting, thus maintaining its propagation. Therefore, the propagation of light from the LD chip 40 is suppressed to 1 / n times that in air throughout the first lens 10.

[0053] In optical device 1, when the refractive index n of the first lens 10 is set to 1.5, light from the optical waveguide 41 can be guided to the second lens 20 with a loss of 0.06 dB and approximately no loss. This reduces the optical power of the second lens 20 and increases the tolerance for alignment misalignment during installation.

[0054] [Second Embodiment]

[0055] In the first embodiment, the optical axis of the optical waveguide 41 is aligned with the optical axis (central axis) of the first lens 10. Therefore, light enters the surface of the first lens 10 perpendicularly, and the reflected light at the surface returns directly in the opposite direction. Consequently, the effect of reflected light towards the optical waveguide 41 sometimes increases. In the second embodiment, to reduce reflected light at the surface of the first lens 10, the optical axis of the first lens 10 is offset from the optical axis of the optical waveguide 41. In the fifth embodiment, where the light travels from the second lens 20 to the first lens 10 (described later), reflected light also occurs at the surface of the first lens 10, causing waveguide interference from optical fibers, etc. Therefore, it is preferable to offset the optical axis of the first lens 10 from the optical axis of the optical waveguide 41.

[0056] Figure 5 Is Figure 1 The diagram shows the optical device with the optical waveguide axis and the optical axis of the first lens misaligned. Figure 5For simplicity, the description of LD chip 40 has been omitted. Figure 5 The optical axis of the first lens 10 is offset parallel to the upper plane of the paper by an axial offset amount d relative to the optical axis of the optical waveguide 41 of the LD chip 40 (not shown). Therefore, in the outgoing light from the first lens, the incident light above the paper at the second lens 20 is not transformed into collimated light but instead undergoes vignetting.

[0057] Figure 6 It means Figure 1 The diagram shows the relationship between the increase in loss and the reflection return rate of the optical device and the axial misalignment between the optical axis of the waveguide and the optical axis of the first lens. Figure 6 The solid line shows the loss increase (dB) relative to the horizontal axis misalignment (μm), and the dashed line shows the reflection return rate (dB) relative to the axis misalignment. Figure 6 The optical waveguide 41, first lens 10, second lens 20, and other elements in the characteristic diagram are the same as those described in the first embodiment. (As per...) Figure 6 As is known, increasing the axial offset d increases the loss but significantly reduces the reflection rate. For example, if the axial offset d is set to 1 μm or more, there is almost no increase in loss, and the reflected light can be suppressed to below -40 dB. Furthermore, considering the relationship with the beam diameter of the optical waveguide 41 of the LD chip 40, offsetting the beam diameter of the optical waveguide 41 by half, i.e., the beam radius ω (0.8 μm in this embodiment), or more, sufficiently suppresses the reflected light.

[0058] [Third Embodiment]

[0059] In the first and second embodiments, the case where the beam radius ω of the optical waveguide 41 is 0.8 μm was described. However, the beam spread angle increases inversely with the beam radius ω. Therefore, according to the structure of the second embodiment, as the beam radius ω decreases, light vignetting and loss also occur at the second lens 20.

[0060] Figure 7 It means in Figure 1 The diagram shows the relationship between the loss caused by vignetting and the beam radius of the optical waveguide when the magnification of the first lens is changed. (Refer to...) Figure 7 For a hemispherical lens with a magnification of 1.5x, the loss increases significantly when the beam radius is below 0.7μm. For example, if the beam radius is 0.5μm, the loss exceeds 1.2dB. On the other hand, it is known that if the magnification is increased to 1.8x or 2.3x, the light spread decreases inversely, thus reducing the loss and enabling the lens to handle even smaller beam radii.

[0061] The magnification of a hemispherical lens is determined by the ratio of its refractive index *n* to the refractive index of air; therefore, a hemispherical lens with a refractive index *n* has a magnification factor of *n*. Thus, the first method to increase magnification is to use a hemispherical lens with a large refractive index *n* in the first lens 10, which increases the magnification proportionally to the refractive index *n*. The second method is to modify the shape of the first lens 10 by setting the radius of curvature *R* of the spherical surface to be smaller than the lens thickness (the length along the optical axis of the lens).

[0062] Figure 8 It means in Figure 1 The diagram shows the characteristics of the optical device, with the lens thickness set to a constant, and the relationship between the magnification and the distance from the front end of the lens to the virtual image relative to the radius of curvature of the hemispherical portion. Figure 9 It means Figure 1 The diagram shows the characteristic relationship between aberration-induced loss and radius of curvature of the optical device. Figure 8 The solid line shows the magnification relative to the lens radius of curvature (μm), and the dashed line shows the distance (μm) from the front of the lens to the virtual image relative to the lens radius of curvature (μm). Figure 10 This is a diagram used to illustrate the relationship between the aberrations and virtual image of the lens when the thickness of the first lens is increased.

[0063] exist Figure 8 The diagram illustrates a case where the thickness D (optical axis length) of the first lens 10 is set to a constant 50 μm, and the radius of curvature R of the hemispherical portion 11 is reduced to increase magnification. When the radius of curvature R is 50 μm, equal to the lens thickness D, the first lens 10 becomes a hemispherical lens. Figure 9 This illustrates the case where the beam radius ω of the optical waveguide 41 is set to a constant 0.5 μm. In the third embodiment, the lens thickness D of the first lens 10 is set to be greater than the radius of curvature R of the hemispherical portion 11, therefore... Figure 10 As shown, the shape of the large lens becomes that of a projectile. Point O, representing the incident position of light from the optical waveguide 41, is a position offset from the center of the hemispherical portion 11 towards the optical waveguide 41. Figure 10 The position of the virtual image is indicated by point P.

[0064] Reference Figure 8 Therefore, it can be seen that in order to set the magnification of the first lens 10 to 2.3x, it is only necessary to set the radius of curvature R to 30μm. In this case, the position P of the virtual image becomes a position 75μm away from the front end of the lens. On the other hand, referring to... Figure 9 The smaller the radius of curvature R becomes, the greater the loss caused by aberrations becomes. If the magnification is 2.3 times (radius of curvature 30μm), the loss becomes 0.29dB, which is greater than the loss caused by vignetting. Figure 10The magnified view shown is a magnified view of the vicinity of the virtual image position, illustrating the case where the virtual image does not converge at point 1 due to aberrations. As described above, in this embodiment, a first lens 10 with a lens thickness greater than the radius of curvature R of the hemispherical portion 11 is used, thus increasing the magnification and enabling the handling of small beam radii ω. However, in this embodiment, the position of the incident light (light source) to the first lens 10 differs from the center position of the lens spherical surface, resulting in aberration-induced losses. The smaller the radius of curvature R becomes, the greater the aberration-induced losses become.

[0065] [Fourth Implementation]

[0066] In order to suppress the loss caused by aberration, an aspherical design is required for the first lens 10. Figure 11 It is used to explain in Figure 1 The diagram shows the optical device, specifically the aspherical lens used in the first lens. In this embodiment, the first lens 10A utilizes the property that, in order to focus light from the light source onto a point 1 of the virtual image, the light path length is equal regardless of the path taken when the light emitted from that point is focused onto that point.

[0067] That is, in Figure 11 When the refractive index of the first lens 10A is set to refractive index n, a predetermined position on the side of the LD chip 40 is set to point O, any position on the surface of the first lens 10A other than the end face of the optical waveguide 41 is set to point S, and the position of the virtual image formed by the first lens 10A is set to point P, there is a relationship that makes the value obtained by subtracting the distance between points S and P from the value obtained by multiplying the distance between points O and S by the refractive index n a constant value. If this relationship is expressed by an equation, it is expressed by the following equation.

[0068] n × distance between points OS - distance between points SP = constant value (1)

[0069] Figure 11The first lens 10A shown illustrates a case where the optical axis of the optical waveguide 41 is orthogonal to the surface of the LD chip 40 side of the first lens 10A at point O, with the virtual image position point P located on the optical axis of the optical waveguide 41. In this case, the optical axis of the first lens 10A coincides with the optical axis of the optical waveguide 41, allowing the lens shape to be formed symmetrically with respect to the optical axis. The first lens 10A is formed as a plane orthogonal to the optical axis of the first lens 10A on the side that joins with the end face of the optical waveguide 41. The area of ​​this plane is smaller than the maximum area of ​​the cross-section of the first lens 10A orthogonal to the optical axis. Thus, the first lens 10A is an aberration-free lens with a magnification of 2.3 times that satisfies the relationship of Equation (1). In the first lens 10A, the loss is 0.09 dB when the beam radius ω of the optical waveguide 41 (not shown) is 0.5 μm, but this loss is caused by vignetting, not by aberration. Furthermore, the light from the optical waveguide 41 does not enter perpendicularly to the lens surface except for the optical axis. Therefore, even without misaligning the optical axis of the first lens 10 with the optical axis of the optical waveguide 41, the reflection return rate is a good -45dB. As described above, the numerical aperture NA of the beam can be reduced in the first lens 10A, thus simplifying the design of lenses after the second lens.

[0070] Figure 12A This diagram illustrates another example of an aspherical lens; lens 10B is shown in relation to... Figure 11 Compared to the first lens 10A shown, the value of the constant value of equation (1) is reduced (increased towards the negative side). Figure 12B The diagram illustrates other examples of aspherical lenses, with the first lens 10C showing the case where the constant value of equation (1) is set to negative infinity. As described above, in this embodiment, various shapes of the first lens can be selected. The first lens 10A of this embodiment has a small effect on reflected light, but as explained by the second embodiment, by misaligning the optical axis of the first lens 10A with the optical axis of the optical waveguide 41, the effect of reflected light from the lens surface can be further suppressed.

[0071] In addition, the aspherical lens shown in this embodiment can also set the position point P of the virtual image to the position separated from the optical axis of the optical waveguide 41. Figure 12C This is a diagram showing other examples of aspherical lenses. Figure 12CThe first lens 10D shown is designed such that the optical axis of the optical waveguide at point O is orthogonal to the surface of the LD chip 40 side of the first lens 10D, and the position P of the virtual image of the first lens 10D is separated from the optical axis of the optical waveguide 41 (Equation 1). Therefore, the straight line (optical axis) connecting the position point O (the position of the light source) of the optical waveguide 41 connected to the first lens 10D and the position point P of the virtual image intersects and is offset from the optical axis of the optical waveguide 41. As described above, in this embodiment, aspherical design can be performed even when the optical axis of the first lens 10D is offset relative to the optical axis of the optical waveguide. As a result, not only is the influence of reflected light suppressed, but aberrations caused by the offset of the optical axis also disappear, and the increase in loss is also suppressed.

[0072] In this embodiment, similar to the second embodiment, the optical axis of the first lens 10A can be offset from the optical axis of the optical waveguide 41. In this case, the optical axis of the optical waveguide 41 is orthogonal to the surface on the LD chip 40 side at a position other than point O of the first lens 10A. Furthermore, in Figure 12C In the case where the virtual image's position P is located at the first lens 10D, which is separated from the optical axis of the optical waveguide 41, the optical axis of the optical waveguide 41 can also be located at a position other than point O of the first lens 10D, which is orthogonal to the surface on the side of the LD chip 40.

[0073] [Fifth Embodiment]

[0074] In the first to fourth embodiments described above, the optical element was described as an LD chip 40. However, in this invention, the optical element can also be a light-receiving element having an optical waveguide. In this case, an optical device with good optical coupling efficiency can be obtained. When the optical element is configured as a light-receiving element, the light travel direction becomes opposite in the optical devices of the first to fourth embodiments. That is, if referring to... Figure 4 The light emitted from the optical fiber (not shown) is transformed into collimated light A by the third lens 30, and then focused by the second lens 20 and directed into the first lens 10. This creates an optical device where the light entering the first lens 10 is focused by the first lens 10 onto the optical waveguide of a light-receiving element (not shown). The light-receiving element with an optical waveguide used in this embodiment can be an optical modulation element, a waveguide-type light-receiving element, or an optical amplification element.

[0075] In this embodiment, an optical system that optically couples an optical element, namely the LD chip 40 and the optical fiber 60, has been described as an example of an optical device. However, as an optical device, it can also be applied to an optical system between optical elements, such as between a light-emitting element and a light-receiving element. In this case, it is sufficient to provide a first lens 10 and a second lens optically coupled to the first lens at the end face of the optical waveguide of the light-emitting element, and also provide a first lens 10 and a second lens optically coupled to the first lens at the end face of the optical waveguide of the light-receiving element, and guide coherent light between the second lenses 20. In addition to resin, glass can also be used as the material for the first lens.

Claims

1. An optical device having: A first lens, which is coupled to the end face of the optical waveguide of the optical element, radiates the emitted light from the optical element; and The second lens is optically coupled to the first lens, converting the outgoing light from the first lens into collimated light. The optical element can be any of a laser element, an optical modulation element, or an optical amplification element. The first lens is an aspherical lens with a curved surface convex to the opposite side of the optical element. The side that engages with the end face of the optical waveguide of the optical element is formed as a plane intersecting the optical axis of the first lens. The area of ​​this plane is smaller than the maximum area of ​​the cross-section of the first lens that is orthogonal to the optical axis of the optical waveguide. When the first lens is set to a refractive index n, a predetermined position on the surface of the optical element side is set to point O, any position on the surface of the first lens other than the surface of the optical element side is set to point S, and the position of the virtual image formed by the first lens is set to point P, the value obtained by subtracting the distance between point S and point P from the value obtained by multiplying the distance between point O and point S by the refractive index n is a constant value.

2. The optical device according to claim 1, wherein, It also has: A third lens, which focuses the collimated light emitted from the second lens; and An optical fiber is optically coupled to the third lens to allow convergent light emitted from the third lens to enter.

3. The optical device according to claim 1 or 2, wherein, The optical axis of the optical waveguide is orthogonal to the surface of the optical element at point O of the first lens, and the position point P of the virtual image of the first lens is located at a position separated from the optical axis of the optical waveguide.

4. The optical device according to claim 1 or 2, wherein, The optical axis of the optical waveguide is orthogonal to the plane on the side of the optical element at a position other than point O.

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