JP2006267284A - Diffraction optical lens, conversion method of optical beam mode profile using the diffraction optical lens, connecting method of optical device and pitch conversion method of the optical device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffraction optical lens in which a mode profile having a certain shape is converted to an optical mode of a different mode profile, and also to provide a mode profile conversion method in which the mode profile of the optical beams is easily converted. <P>SOLUTION: In the diffraction optical lens, a plurality of prisms having different angles is formed into a plurality of elliptical shapes which have the same center and respectively have similarity relationships and the prisms function as a bent surface. In the optical beam mode profile conversion method, first optical beams having an elliptically shaped mode profile are converted into second optical beams having a mode profile of the elliptical shape in which the ratio between the major axis and the minor axis is made different with respect to the elliptical shape of the first optical beams. In the conversion method, the diffraction optical lens is used to convert the first optical beams into the second optical beams. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a diffractive optical lens, a method for converting a mode profile of a light beam using the same, a method for connecting an optical device, and a pitch converting method for an optical device.

  Optical devices are widely used in daily life from data retrieval of optical media by an optical pickup in a computer to a telecommunication network of a data transmission system. In these systems, it is necessary to combine two or more devices without optical loss. For example, an optical fiber, which is indispensable in a network, can transmit data over a long distance with a very small propagation loss, but has a large loss when coupling optical devices in a system. Therefore, in an optical transmission system, coupling loss is key to minimizing the budget.

  Typically, coupling loss occurs due to the different optical mode profiles between the two devices. For example, when a single mode fiber (SMF) is connected to a single mode optical waveguide, the single mode fiber has a circular mode profile, whereas the single mode optical waveguide has an almost elliptical mode profile. The difference between the shape of the mode profile and the optical power causes a large coupling loss. Therefore, it is preferable to minimize the mode difference or convert from one mode to another.

  A so-called butt coupling is known as a technique for connecting the end of an optical fiber and an optical waveguide without being aware of different modes. Thereby, the two optical devices can be easily connected at low cost, but cause a high coupling loss of 4 dB to 6 dB per surface. The cause of the coupling loss is mainly that the mode profile of the optical fiber is circular, whereas the optical waveguide is almost elliptical in the vertical direction shorter than the horizontal direction in most cases. As a technique for suppressing the coupling loss, there is a technique for correcting the mode profile of the optical waveguide so as to be as close to the mode of the optical fiber as possible. This technique is generally used for a diffusion optical waveguide, and is performed by precisely controlling the formation of a titanium film widely used in lithium niobate optical waveguides. This method is a simple method for reducing the coupling loss, but the method can be applied only to a diffusion optical waveguide and is limited in other optical waveguides.

Therefore, another method is adopted as a method for reducing the coupling loss. (1) A tapered optical waveguide is used to convert the mode profile. (2) A micro optical element is disposed between the optical fiber and the optical waveguide, and a light beam is directed from the optical fiber toward the optical waveguide. Condensing and transforming the shape. In (1), a precise manufacturing scheme is required to form a tapered shape (see, for example, Non-Patent Document 1). In particular, when a loss that causes variations in the taper shape is considered, in most cases, not only the chemical material of the taper material but also the optical material is limited to reduce the coupling loss, which eventually leads to incomplete manufacturing. In (2), various micro optical elements are sandwiched between an optical fiber and an optical waveguide. In addition, for example, an example in which the optical fiber itself is tapered so as to be adapted to the optical waveguide (see, for example, Non-Patent Document 2) is also included. However, in this case, the allowable range in which the tapered optical fiber and the optical waveguide are compatible is narrow, and in most cases only a circular shape can be achieved.
A similar approach is to use a spherical lens fiber to focus the light beam on the optical waveguide core. However, it is limited only to form a circular mode profile, and there is a problem of the trade-off relationship between the coupling loss and the tolerance as described above. Another method for converting a circular mode profile into an elliptical mode profile is a method using a GRIN lens (see, for example, Non-Patent Document 3). Although this technique is useful, using a GRIN lens has a large convertible mode profile, cannot produce a light beam with a small diameter, and does not work well for combining single mode optical waveguides.

  On the other hand, examples in which a conventional Fresnel lens is used to improve coupling loss are known (see, for example, Patent Documents 1 and 2). However, the Fresnel lens has a configuration in which prisms are concentrically arranged, is symmetric in both the vertical direction and the horizontal direction, and cannot convert the mode profile only by converting the size of the light beam.

As described above, conventionally, there has been no mode conversion that simultaneously realizes conversion of one mode profile to another mode profile within an allowable range and reduction of coupling loss.
US Pat. No. 5,359,684 US Pat. No. 5,513,289 Min-Cheol Oh et al, IEEE Photonics Tech.Lett., 14, pp.1121-1123 (2002) Thomas Paatzsch et al, Appl. Opt., 36, pp. 5129-5133 (1997) Shifu Yuan et al, Appl. Opt., 38, pp. 3214-3222 (1999)

An object of the present invention is to solve the conventional problems and achieve the following objects. That is,
An object of the present invention is to provide a diffractive optical lens that can convert a mode profile of one shape into an optical mode of another mode profile, a mode profile conversion method that can easily convert a mode profile of a light beam, and optical in different modes. An object of the present invention is to provide an optical device connection method capable of connecting devices with a small coupling loss, and a conversion method capable of easily converting the pitch of the optical device.

Means for solving the problems are as follows. That is,
<1> A diffractive optical lens, wherein a plurality of prisms having different angles are formed in a plurality of elliptical shapes having the same center and similar relationships, and the prism functions as a bent surface.

<2> The first light beam having an elliptical mode profile is changed to a second light beam having an elliptical mode profile in which the ratio of the major axis to the minor axis is different from the elliptical shape of the first optical beam. A method of converting a mode profile of a light beam to be converted, wherein the conversion from the first light beam to the second light beam is performed using the diffractive optical lens according to <1>. This is a method of converting a mode profile of a light beam.

<3> An optical device connection method for connecting two types of optical devices having different mode profiles, each of the two types of optical devices being an optical fiber, and a fiber mode of one optical fiber and a fiber mode of the other optical fiber. Are connected using the diffractive optical lens described in <1> above, and the two types of optical devices are connected.

<4> An optical device connection method for connecting two types of optical devices having different mode profiles, wherein one of the two types of optical devices is an optical fiber and the other is an optical waveguide. An optical device connection method comprising connecting an optical fiber and an optical waveguide by matching a profile with a mode profile of the optical waveguide using the diffractive optical lens according to <1>.

<5> Two or more of the two types of optical devices are arranged in an array, and the two types of optical devices arranged in the array are connected. <3> or <4> It is the connection method of the optical device as described in above.

<6> A pitch conversion method for an optical device that converts the pitch of two or more optical devices on the input side to a different pitch on the output side, wherein the diffractive optical lens according to <1> The pitch conversion method of the optical device is characterized in that the pitch conversion is performed by arranging the optical axis and the central axis of the optical device so as not to coincide with each other.

  According to the present invention, a diffractive optical lens that can convert a mode profile of one shape into an optical mode of another mode profile, a mode profile conversion method that can easily convert a mode profile of a light beam, and optical in different modes It is possible to provide a method for connecting an optical device to which a device can be connected and a conversion method capable of easily converting the pitch of the optical device.

The diffractive optical lens of the present invention is characterized in that a plurality of prisms having different angles are formed in a plurality of elliptical shapes having the same center and similar relationships, and the prism functions as a bending surface.
Also, the mode profile conversion method of the present invention provides a first light beam having an elliptical mode profile in an elliptical mode in which the ratio of the major axis to the minor axis is different from the elliptical shape of the first light beam. A method for converting a mode profile of a light beam to be converted into a second light beam having a profile, wherein the conversion from the first light beam to the second light beam is performed using the diffractive optical lens of the present invention. It is characterized by doing.

  In other words, the diffractive optical lens of the present invention is a lens in which a concentric prism of a planar plate lens known as a Fresnel lens is changed to an ellipse. By applying the diffractive optical lens of the present invention to an optical waveguide, conversion of different mode profiles can be easily performed. When two optical devices are coupled, the coupling loss is reduced and the allowable range of adaptation is wide. Both can be achieved. In particular, the diffractive optical element and the optical waveguide can be easily connected.

  FIG. 1 is a diagram for explaining a diffractive optical lens of the present invention. In the diffractive optical lens 10 shown in FIG. 1, a plurality of prisms having different angles are arranged in an elliptical shape with different asymmetric shapes in the horizontal direction and the vertical direction. More specifically, the diffractive optical lens 10 is formed with an elliptical prism whose diameter in the horizontal direction is longer than that in the vertical direction. Is formed so that the angle increases as the distance from the center decreases. FIG. 1 shows a state where a circular mode profile light beam emitted from the optical fiber 12 is transmitted through the diffractive optical lens 10 and converted into an elliptical mode profile light beam 14 whose diameter in the horizontal direction is longer than that in the vertical direction. ing.

  In the diffractive optical lens of the present invention, the angle of the prism is larger in the vertical direction than in the horizontal direction, so that the vertical direction is more refracted. Therefore, the beam diameter is smaller in the vertical direction than in the horizontal direction. Similarly, since the angle of the prism is small in the horizontal direction, refraction is small and the beam diameter is large compared to the vertical direction. In the diffractive optical lens of the present invention, a light beam having a circular mode profile is converted into an ellipse from the above principle.

  In FIG. 2, an elliptical light beam (first light beam) 16 is diffracted from the elliptical light beam (second light beam) using the diffractive optical lens 10 in a horizontal direction and a vertical direction. The light beam is converted to (18). That is, not only the circular light beam but also the elliptical light beam is converted according to the ratio between the major axis and the minor axis of the elliptical shape of the prism of the diffractive optical lens 10.

  FIG. 3 shows how the mode profile of a light beam is converted by different diffractive optical lenses. A is the shape before entering the diffractive optical lens (circular: 10.4 μm (horizontal direction) × 10.4 μm (vertical direction)). The incident light is shaped into B, C, and D by the diffractive optical lenses 20, 22, and 24, respectively. The diffractive optical lens 20 has the same diameter ratio in the horizontal and vertical directions of the prism, that is, a conventional Fresnel lens, and is not a diffractive optical lens of the present invention. The diffractive optical lenses 22 and 24 are diffractive optical lenses of the present invention. When the light beam before incidence passes through the diffractive optical lenses 20, 22, and 24, the respective beam diameters (horizontal direction × vertical direction) are 9.0 μm × 9.0 μm (A), 9.0 μm × 5.2 μm. (B), 9.0 μm × 3.8 μm (C). The outer shape of each diffractive optical lens is a plate-like square of 25 μm × 25 μm.

FIG. 4 is a view showing a state in which the diffractive optical lens of the present invention is attached to the end face of the optical fiber 26. As shown in FIG. 4, the optical fiber 26 has a two-layer configuration including a clad 28 on the outer side and a core 30 on the inner side, and circular diffraction in which elliptical prisms having different diameters in the horizontal direction and the vertical direction are formed on the end surface. An optical lens 32 is attached. In FIG. 4, the diameter of the diffractive optical lens 32 is 25 μm.
Although FIG. 4 shows a form in which the diffractive optical lens 32 is attached to the end face of the optical fiber 26, the end face of the optical fiber 26 is etched so that an elliptical prism similar to the surface of the diffractive optical lens 32 is directly formed. It may be formed.

  FIG. 5 is a diagram illustrating a state in which the optical fiber 26 illustrated in FIG. 4 is connected to the optical waveguide. In FIG. 5, the optical fiber 26 and the optical waveguide 34 are arranged so that the light beam emitted from the optical fiber 26 enters the core 36 of the optical waveguide 34. A diffractive optical lens 32 is attached to the end surface of the optical fiber 26 (the surface on the optical waveguide 34 side), and the mode profile of the optical fiber 26 is converted by the diffractive optical lens 32 so that it matches the mode profile of the optical waveguide 34. Is set to That is, the mode profile of light propagating in the optical fiber 26 is circular, but is converted into an elliptical shape by the diffractive optical lens 32 located on the end face, and coincides with the optical waveguide 32. Therefore, the coupling loss can be reduced.

  FIG. 6 is a cross-sectional view showing details of the optical waveguide 34 shown in FIG. The optical waveguide 34 has a lower electrode 40, a lower cladding layer 42, a core 36, an upper cladding layer 44, and an upper electrode 46 on a substrate 38. The core 36 is a ridge type as shown in FIG. An example of numerical values for operating in the single mode of this configuration to minimize loss due to absorption of optical power is shown below. The lower cladding layer 42 and the upper cladding layer 44 have a refractive index of 1.5128 for light having a wavelength of 1.55 μm, and the core 36 has a refractive index of 1.5886 for the same light. The thicknesses of the lower clad layer 42 and the upper clad layer 44 are both 4.5 μm, and the thickness of the core 36 is 1.6 μm at the ridge portion, and is 1.2 μm otherwise. The width of the ridge portion is 5.00 μm. And the absorption between both poles on the above conditions was less than 0.1 dB / cm.

  FIG. 7 is a graph showing the coupling loss corresponding to the cases A to D shown in FIG. 3 when the optical fiber is connected to the optical waveguide as shown in FIG. A is the case of direct propagation from the optical fiber, and the coupling loss is 3.8 dB. B is a case where a conventional Fresnel lens is transmitted, and the coupling loss is reduced to 3 dB. As described in the explanation of FIG. 3, C is a case where the light is transmitted through the diffractive optical lens of the present invention, and the coupling loss is reduced to 1.4 dB. Similarly, for D, the coupling loss is reduced to 0.7 dB. That is, when the diffractive optical lens of the present invention is used, the coupling loss is improved by 3.1 (= 3.8−0.7) dB. It can be said that the optical power of about twice that of the prior art has been converted by the present invention.

  Next, the coupling loss when the optical waveguide shown in FIG. 5 is used and the elliptical shapes of the light beams converted according to the present invention are different will be considered. FIG. 8 is a graph showing the coupling loss when the diffractive optical lens of the present invention is designed so that the elliptical shape of the light beam output after the mode profile is converted is different. The three graphs in FIG. 8 show the coupling loss (y-axis) relative to the vertical diameter (x-axis) for elliptical light beams with horizontal diameters of 9.0 μm, 7.5 μm, and 6.0 μm, respectively. ). FIG. 8 shows that the coupling loss changes more greatly when the vertical diameter is changed than when the elliptical horizontal diameter is changed. Further, from FIG. 8, even when the horizontal diameter changes by 3 (= 9.0-6.0) μm and the vertical diameter changes by 1.4 μm, the coupling loss is suppressed to less than 0.5 dB. I understand. Theoretically, it can also be seen that a coupling loss of 0 dB can be achieved when mode conversion into an elliptical light beam of 6.0 μm × 2.5 μm is performed.

  The above-described diffractive optical lens of the present invention can be manufactured by binary optics and holographic recording by applying a method of manufacturing a Fresnel lens. For details on binary optics, see Y.K. Orihara et al., Appl. Opt. 40, pp. Details of holographic recording described in 5877-5855 can be found in Anthony VanderLuggt, Optical Signal Processing, John Wiley & Sons, Inc. 1992, p.82. Examples of the material of the diffractive optical element include transparent acrylic resin, silica, and epoxy resin.

  The shape and size of the diffractive optical lens of the present invention are not particularly limited. For example, the length of one side can be a square of 5 to 40 μm or a circle of 2 to 20 μm in diameter. Moreover, thickness can be 50-5000 micrometers.

  The present invention can also be applied to an arrayed optical device. FIG. 11 is an example showing a state in which n × m diffractive optical lenses are arranged. In this way, the optical device array that is freely arranged can be connected with a small deficiency by arranging a diffractive optical lens with an appropriate degree of conversion.

  FIG. 12 shows a state in which an optical fiber array is configured by arranging n × m (3 × 4) optical fibers 26. A diffractive optical lens is attached to the end face of each optical fiber. According to this aspect, the diffractive optical lens attached to each optical fiber can be connected with a small coupling defect by adapting to the mode profile of the optical waveguide array to be connected.

  Next, the pitch conversion method for the optical device array of the present invention will be described. First, the principle will be described with reference to FIG. FIG. 9 is a diagram corresponding to FIG. 1, and the same components as those in FIG. 1 are denoted by the same reference numerals. In FIG. 9A, as shown in FIG. 1, the optical axis of the optical fiber 12 coincides with the center of the diffractive optical lens 10, and the light beam travels in the X direction. On the other hand, in FIG. 9B, the optical axis of the optical fiber 12 is shifted so as not to coincide with the center of the diffractive optical lens 10. The refractive index of the diffractive optical lens 10 increases as the distance from the center increases. Therefore, when the diffractive optical lens 10 is disposed as shown in FIG. . In the pitch conversion method of the optical device array of the present invention, the pitch conversion is performed by utilizing the deviation of the optical axis. The principle shown in FIG. 9B is useful when it is necessary to shift the optical axis and perform focusing simultaneously.

  FIG. 10 shows how the pitch is converted using the two optical fibers 12A and 12B. In FIG. 10, the optical fiber 12A and the optical fiber 12B are arranged at a pitch a. The optical path of the optical fiber 12A is shifted from the center of the diffractive optical lens 10 so that the light beam is transmitted through the lower half, while the optical path of the optical fiber 12B is shifted from the center of the diffractive optical lens 10 and the light beam is transmitted through the upper half. Are arranged to be. The optical axis of the optical beam from the optical fiber 12A is shifted upward, and the optical axis of the optical beam from the optical fiber 12B is shifted downward. Therefore, the mode profile is converted by 10 and the pitch is shifted by b after the conversion. According to the pitch conversion method of the optical device array of the present invention, the pitch can be converted into various pitches only by changing the arrangement of the diffractive optical lenses, and the coupling loss can be reduced. The pitch can be changed by changing the position of the diffractive optical lens.

It is a figure which shows a mode that it converts into a elliptical mode profile from a circular mode profile. It is a figure which shows a mode that it converts into the elliptical mode profile from which an elliptical mode profile differs in ratio of a major axis and a minor axis. It is a figure which shows a mode that the circular mode profile was converted into the mode profile of another shape using the plate-shaped optical element of this invention. It is a figure which shows the state which attached the plate-shaped optical element of this invention to the end surface of the optical fiber. It is a figure which shows the state which connected the optical fiber shown in FIG. 4 to the optical waveguide. It is sectional drawing of the optical waveguide shown in FIG. It is a figure which shows the coupling loss at the time of the mode profile conversion of AD shown in FIG. 2 with a graph. It is a figure which shows the change of the coupling loss with respect to various elliptical shapes by a graph. It is a figure which shows a mode that the laser beam was advanced straight using the plate-shaped optical element of this invention, and a mode that the optical axis was shifted. It is a figure explaining that the pitch conversion of two optical devices is possible with the plate-shaped optical element of this invention. It is a figure which shows the nxm multi-channel optical device array to which this invention is applied. It is a figure which shows the nxm optical fiber array to which this invention is applied.

Explanation of symbols

10 22 24 32 Diffractive optical lens 12 26 Optical fiber 34 Optical waveguide

Claims (6)

  A diffractive optical lens characterized in that a plurality of prisms having different angles are formed in a plurality of elliptical shapes having the same center and similar relationships, and the prism functions as a curved surface. Light for converting a first light beam having an elliptical mode profile into a second light beam having an elliptical mode profile in which the ratio of the major axis to the minor axis is different from the elliptical shape of the first optical beam. A beam mode profile conversion method comprising:
The method of converting a mode profile of a light beam, wherein the conversion from the first light beam to the second light beam is performed using the diffractive optical lens according to claim 1. An optical device connection method for connecting two types of optical devices having different mode profiles,
The two optical devices are both optical fibers, and the fiber mode of one optical fiber and the fiber mode of the other optical fiber are matched using the diffractive optical lens according to claim 1, and the two optical devices are used. To connect the optical device. An optical device connection method for connecting two types of optical devices having different mode profiles,
One of the two types of optical devices is an optical fiber and the other is an optical waveguide, and the mode profile of the optical fiber and the mode profile of the optical waveguide are matched using the diffractive optical lens according to claim 1. An optical device connection method comprising connecting an optical fiber and an optical waveguide.   5. The optical device according to claim 3, wherein two or more of the two types of optical devices are arranged in an array, and the two types of optical devices arranged in the array are connected. Connection method. A pitch conversion method for an optical device that converts the pitch of two or more optical devices on the input side to a different pitch on the output side,
A pitch conversion method for an optical device, wherein the pitch conversion is performed by arranging the diffractive optical lens according to claim 1 so that an optical axis of the diffractive optical lens does not coincide with a central axis of the optical device. .