JP2004157530A - Optical module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical module which satisfies three conditions of the suppression of the leakage of waveguide light to a silicon substrate, a mode-field diameter capable of efficient connection with an ordinary single mode fiber and the formation of a single mode condition at the same time. <P>SOLUTION: A perfectly embedded optical waveguide is provided with a silicon substrate 20, an underclad 21 made of quartz laminated on the silicon substrate 20, an overclad 23 laminated on the underclad 21 and a core 22 which is laminated on the underclad 21 and is surrounded by the overclad 23 in three directions of side surfaces and upper surface. Therein, the difference of relative refraction index between the core 22 and the underclad 21 is made larger than that between the core 22 and the overclad 23. <P>COPYRIGHT: (C)2004,JPO

Description

  The present invention relates to an optical module such as an optical waveguide filter used in the field of optoelectronics and optical communication.

  Research and development of Si wire waveguides and photonic crystal waveguides using SOI (Silicon On Insulator) substrates have been conducted with the aim of miniaturization of optical circuits. Since it is on the order of microns, connection must be made via a polymer waveguide or the like for connection with an optical fiber or the like (for example, Non-Patent Document 1).

  FIG. 4A is a cross-sectional view of a conventional optical module formed on an SOI substrate, and FIG. 4B is a side view of the optical module of FIG. As shown in the figure, 10 is a silicon substrate, 11 is an under clad made of quartz, 12 is a core made of polymer, and 13 is a clad made of polymer. The thickness of the under cladding 11 is 3 μm, and its refractive index is 1.49.

The applicant has not found any prior art documents related to the present invention other than the prior art documents specified by the prior art document information described in this specification by the time of filing.
Shoji et al., "External Coupling Structure of Si Wire Optical Waveguide Formed on SOI Substrate," Proceedings of Spring Conference, Japan Society of Applied Physics, 2001, No. 3, 30a-YK-11

  The mode field diameter of a normal single mode fiber is about 9 μm, and a mode field diameter of 5.5 μm or more is required at the connection with the optical fiber to connect the planar waveguide to the optical fiber with a loss of 1 dB or less. It is.

  In addition, the thickness of the under cladding used in a normal quartz optical waveguide is sufficiently large, about 20 μm, and there is no need to consider leakage of guided light to the Si substrate. However, in an optical waveguide using silicon oxide of an SOI substrate, that is, quartz as an under cladding, it is difficult to make the quartz layer thicker than 3 μm due to a problem in a manufacturing process of the SOI substrate. Therefore, when the relative refractive index difference between the under cladding and the core is 1% or less, as set in a normal optical waveguide, there is a problem that the guided light leaks to the silicon substrate through the under cladding.

  FIG. 5 shows a completely buried optical waveguide in which the refractive index of a square core having a core section of 7 μm square is 1.47 and the refractive index of the cladding is 1.462, and a quartz layer 3 μm from the end of the core. A silicon layer with a thickness of 0.3 μm was provided, imitated on a silicon substrate, and the light intensity distribution when a TM-polarized light beam having a wavelength of 1.55 μm was incident on the core and propagated at 100 μm was shown by a mode solver. It is.

  As can be seen from this figure, light is also propagated to the silicon layer imitating the silicon substrate, and even with a wiring of only about 100 μm, the coupling with the silicon layer is not negligible, and it cannot be practically used. it is obvious.

  In the waveguide having such a configuration, in a 1.55 μm band mainly used as a band for optical communication, when the refractive index of quartz is 1.46, in order to prevent light from leaking to the silicon substrate, It is necessary that the refractive index of the core 12 is 1.49 or more (“Optical Integrated Circuit”, Ohmsha, Hiroshi Nishihara and others). However, if the refractive index of the core 12 is 1.49 or more, the relative refractive index difference between the quartz layer and the core is 2% or more. Therefore, the core size satisfying the single mode condition is about 3 μm or less on one side. There is a need to. For this reason, conventionally, it is necessary to use a fiber having a small mode field diameter, which has been subjected to TEC processing, connected to a normal single mode fiber, and the like.

  SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object of the present invention is to prevent leakage of guided light to a silicon substrate in an optical module formed on an SOI substrate, and to realize a single-mode fiber. It is an object of the present invention to provide an optical module that simultaneously satisfies three conditions, that is, a mode field diameter that can be connected efficiently and a single mode condition.

The optical module of the present invention has a silicon substrate, an under clad disposed on the silicon substrate, a core made of silicon having a rectangular cross section disposed on the under clad, and disposed so as to cover the core. An over cladding, wherein the under cladding, the core, and the over cladding covering the side and top surfaces of the core constitute a completely buried optical waveguide, and have a refractive index of the over cladding and a refractive index of the under cladding. It is something larger than.
Further, in the optical module of the present invention, a relative refractive index difference between the core and the under cladding is larger than a relative refractive index difference between the core and the over cladding.

Further, the optical module of the present invention comprises a silicon substrate, an under cladding disposed on the silicon substrate, a first core disposed on the under cladding, made of silicon having a rectangular cross section, And a taper portion made of silicon having a cross section gradually decreasing toward the tip and made of silicon, and a second taper portion arranged so as to cover the taper portion. 2 core, and an over cladding disposed to cover the tapered portion and the second core, wherein the under cladding and the first core constitute a first optical waveguide, The undercladding, the tapered portion, the second core, and the overcladding form a mode field size converter, and the undercladding and the second core Wherein the over-cladding, the second optical waveguide configured, the refractive index of the over cladding is made larger than the refractive index of the under-cladding.
Further, in the optical module of the present invention, a relative refractive index difference between the second core and the under cladding is larger than a relative refractive index difference between the second core and the over cladding.

  According to the present invention, in an optical module having an optical waveguide of a completely buried type, by making the refractive index of the over cladding larger than the refractive index of the under cladding, or by making the relative refractive index difference between the core and the under cladding relative to the core. By making the relative refractive index difference larger than that of the over cladding, even if the silicon oxide film of the SOI substrate is used as the under cladding, the leakage of the guided light to the silicon substrate can be prevented while maintaining the guided light in a single mode. Since the diameter of the core can be increased and the cross-sectional dimension of the core can be increased, it is possible to efficiently couple with a normally used single mode fiber having a mode field diameter of about 9 μm.

  According to the present invention, in the optical module having the first optical waveguide, the mode field size converter, and the second optical waveguide, the refractive index of the over cladding is made larger than the refractive index of the under cladding, or By making the relative refractive index difference between the second core and the under cladding larger than the relative refractive index difference between the second core and the over cladding, the waveguide light can be obtained even when the silicon oxide film of the SOI substrate is used as the under cladding. Can be prevented from leaking guided light into the silicon substrate while maintaining the single mode, and the cross-sectional dimension of the second core can be increased. It is possible to efficiently couple with the fiber.

[First Embodiment]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1A is a sectional view of an optical module according to a first embodiment of the present invention, and FIG. 1B is a side view of the optical module of FIG. In the drawing, reference numeral 20 denotes a silicon substrate, 21 denotes a flat undercladding made of a silicon oxide film as a whole, 22 denotes a core having a substantially rectangular cross section made of a polymer such as epoxy resin or polyimide, and 23 denotes an epoxy resin or polyimide. Overcladding made of polymer. w1 represents the width of the core 22 and w2 represents the thickness of the core 22. The thickness of the under cladding 21 is 3 μm. In addition, the silicon substrate 20 and the under cladding 21 each constitute a part of the SOI substrate.

  In the present embodiment, the refractive indices of the core 22, the under cladding 21, and the over cladding 23 are 1.5, 1.46, and 1.49, respectively, and the relative refractive index difference between the core 22 and the under cladding 21 ((core 22-the refractive index of the under cladding 21) / the refractive index of the core 22), the relative refractive index difference between the core 22 and the over cladding 23 ((the refractive index of the core 22-the refractive index of the over cladding 23) / the core 22 Are 2.7% and 0.7%, respectively. If the relative refractive index difference between the core 22 and the under cladding 21 is 1% or less of a normal value, the guided light leaks to the under cladding 21, so that 1% or more or 1.5% or more is preferable.

  The problem of the leakage of the guided light to the under cladding 21 can also be avoided by controlling the width and thickness of the cross-sectional shape of the core 22. For example, the width w1 and the thickness w2 of the cross section of the core 22 are preferably set to values in the range of 5.5 to 9 μm, and the width w1 and the thickness w2 are more preferably equal. In the present embodiment, the width w1 and the thickness w2 of the cross section of the core 22 are both 7 μm.

  FIG. 2 shows the light intensity distribution after 1.55 μm TM-polarized light is incident on the optical waveguide having the cross-sectional structure of the present embodiment and propagated by 10 mm, using a mode solver. . As can be seen from FIG. 2, light is not propagated to the silicon layer, and the leakage of light to the silicon substrate is prevented while maintaining a substantially circular mode profile of 7 μm or more.

  With the above configuration using a material having a refractive index larger than the refractive index of the under cladding 21 for the over cladding 23, leakage of guided light to the under cladding 21 can be prevented. In addition to satisfying the single mode condition of the guided light, a mode field diameter required to be efficiently connected to the single mode fiber can be realized.

[Second embodiment]
Next, a second embodiment of the present invention will be described. FIG. 3A is a plan view of an optical module including a mode field size conversion unit according to a second embodiment of the present invention, and FIG. 3B is a line AA of the optical module of FIG. It is sectional drawing. In the figure, 30 is a first optical waveguide, 31 is a mode field size converter, 32 is a second optical waveguide connected to the first optical waveguide 30, 33 is a silicon substrate, and 34 is formed on a silicon substrate. Undercladding made of a silicon oxide film, 35 is an overcladding made of a material such as a polymer, 36 is a first thin wire core made of silicon formed on the undercladding 34, and 37 is a first core. A tapered portion made of silicon, which is an end portion of the first core, is a second core made of a polymer at least partially disposed on the tapered portion 37 of the first core. The main part of the second core 38 constitutes a second optical waveguide together with the over cladding 35 and the under cladding 34. The tapered portion 37 is formed so that the cross-sectional area gradually decreases toward the distal end of the first core 36.

The first optical waveguide 30 and the second optical waveguide 32 are formed using a silicon substrate 33 and an under clad 34 as a common substrate, and are optically connected via a mode field size converter 31.
The field size converter 31 is constituted by an under clad 34, a tapered portion 37, a second core 38 disposed on the tapered portion 37, and an over clad 35 disposed on and around the second core. I have. The first optical waveguide 30 includes an under cladding 34, a core 36 disposed thereon, and an over cladding 35. In the first optical waveguide 30, the over cladding 35 is not always necessary. The reason is that the first optical waveguide 30 having a silicon core as a core has strong light confinement, and does not leak light to the lower substrate even when there is no over cladding.

  When transmitting light in the 1.55 μm band, which is most often used for optical communication, the core 36 of the first optical waveguide 30 has a cross-section thickness and width of about 0.3 μm, and the taper portion 37 has a length. Is 300 to 1000 μm, and the width of the tip is about 0.1 μm. The core 38 of the second optical waveguide 32 has a cross section thickness and width of about 7 μm, and a refractive index of 1.53. The over cladding 35 common to the first optical waveguide 30, the mode field size converter 31, and the second optical waveguide 32 has a refractive index higher than that of the under cladding 34 and lower than that of the second core 38 by about 1.51. Consisting of a polymer of

  Next, a light propagation state in the optical module of the present embodiment will be described. The light incident from the left end face of the first core 36 of the first optical waveguide 30 shown in FIGS. 3A and 3B propagates through the core 36 and then tapers in the mode field size converter 31. It reaches the left end position of the part 37. As the light propagates through the tapered portion 37 to the right in FIG. 3A, the core width gradually decreases, the confinement of the light weakens, and the mode field tends to spread to the surroundings. However, at this time, since the second core 38 having a higher refractive index than the under clad 34 exists adjacently, the distribution of the optical power varies from the first core 36 of the first optical waveguide 30 to the second optical waveguide 32. It gradually moves to the second core 38.

  Conversely, when light enters from the right end of the second core 38 of the second optical waveguide 32 shown in FIGS. 3A and 3B, the light travels from right to left. Then, the light distribution moves to the first core 36 of the first optical waveguide 30 via the second core 38 and the tapered portion 37. In this way, by connecting the first core 36 of the first optical waveguide 30 and the second core 38 of the second optical waveguide 3 via the tapered portion 37, the mode field size (diameter) with high efficiency is improved. ) Conversion can be realized.

The optical module having the first optical waveguide 30, the mode field size converter 31, and the second optical waveguide 32 shown in FIGS. 3A and 3B is manufactured as follows.
First, an SOI substrate including a silicon substrate 33, an overall flat undercladding 34 made of a silicon oxide film formed on the silicon substrate 33, and a silicon layer formed on the undercladding 34 is prepared. The silicon layer on the clad 34 is processed by electron beam lithography and etching to form a first core 36 and a tapered portion 37. Next, a polymer material to be the second core 38 is applied on the substrate, and the polymer material is processed by photolithography and etching to form the second core 38. Finally, a polymer to be the overcladding 35 is applied to the whole to complete the optical module.

  In the present embodiment, the refractive indices of the second core 38, the under cladding 34 made of a silicon oxide film, and the over cladding 35 are 1.52, 1.46, and 1.51, respectively. The relative refractive index difference from the cladding 34 ((the refractive index of the core 38-the refractive index of the under cladding 34) / the refractive index of the core 38), the relative refractive index difference between the second core 38 and the over cladding 35 ((core 38 The refractive index of the over clad 35) / the refractive index of the core 38) is 3.9% and 0.7%, respectively. As described above, the refractive index of the over cladding 35 is larger than the refractive index of the under cladding 34, and the relative refractive index difference between the second core 38 and the under cladding 34 is changed to the relative refractive index between the second core 38 and the over cladding 35. By making the difference larger than the difference, it is possible to prevent the light propagating through the mode field size converter 31 and the second optical waveguide 32 from leaking to the silicon substrate 33, and as a result, the loss of the optical module can be reduced.

  At the same time, in the configuration of the present embodiment, the relative refractive index difference between the second core 38 and the over clad 35 is set to 0.7%, so that the thickness of the cross section of the second core 38 while satisfying the single mode condition is satisfied. And the width can be increased to 7 μm, and the optical module and the single mode fiber can be efficiently connected.

  In the present embodiment, the side surface and the upper surface of the first core 36 of the first optical waveguide 30 are covered with the over cladding 35 having a refractive index higher than that of the under cladding 34 by about 3%. 5 and the under cladding 34 having a refractive index of 1.46, and the relative refractive index difference between silicon and the over cladding 35 having a refractive index of 1.51 are both very large. There is no effect on the light confinement characteristics of the wave path 30, and there is no effect on bending loss or propagation loss.

  In the first and second embodiments, a polymer is used as a material for the over claddings 23 and 35 and the cores 22 and 38, but light is transmitted and the first and second refractive indexes are adjusted by, for example, doping. As long as the material can be equivalent to that of the second embodiment, not only polymers such as epoxy and polyimide but also inorganic materials such as silicon oxide and silicon nitride oxide (SiON) can be used as materials for the over claddings 23 and 35 and the cores 22 and 38. Can be used as

  The present invention can be applied to the field of optoelectronics and the field of optical communication.

FIG. 2 is a cross-sectional view and a side view of the optical module according to the first embodiment of the present invention. FIG. 3 is a diagram illustrating a light intensity distribution according to the first embodiment of the present invention. It is the top view and sectional drawing of the optical module used as the 2nd Embodiment of this invention. It is a sectional view and a side view of a conventional optical module formed on an SOI substrate. FIG. 11 is a diagram illustrating a light intensity distribution in a conventional optical module.

Explanation of reference numerals

20 silicon substrate, 21 under clad, 22 core, 23 over clad, 30 first optical waveguide, 31 mode field size converter, 32 second optical waveguide, 33 silicon substrate, 34 Under cladding, 35 over cladding, 36 first core, 37 taper, 38 second core.

Claims (4)

A silicon substrate,
An under clad arranged on this silicon substrate,
A core made of silicon having a cross section arranged on the under cladding,
With an over clad arranged to cover this core,
The under cladding, the core, and the over cladding covering the side and top surfaces of the core constitute a completely embedded optical waveguide,
An optical module, wherein the refractive index of the over cladding is larger than the refractive index of the under cladding. A silicon substrate,
An under clad arranged on this silicon substrate,
A core made of silicon having a cross section arranged on the under cladding,
With an over clad arranged to cover this core,
The under cladding, the core, and the over cladding covering the side and top surfaces of the core constitute a completely embedded optical waveguide,
An optical module, wherein a relative refractive index difference between the core and the under cladding is larger than a relative refractive index difference between the core and the over cladding. A silicon substrate,
An under clad arranged on this silicon substrate,
A first core made of silicon having a rectangular cross section disposed on the under cladding;
A taper portion made of silicon having a cross section whose cross-sectional area gradually decreases toward the tip, which is arranged integrally with the end of the first core on the under clad,
A second core disposed so as to cover the tapered portion;
An over clad disposed so as to cover the tapered portion and the second core,
The undercladding and the first core constitute a first optical waveguide,
The undercladding, the tapered portion, the second core, and the overcladding form a mode field size converter,
The under cladding, the second core, and the over cladding constitute a second optical waveguide,
An optical module, wherein the refractive index of the over cladding is larger than the refractive index of the under cladding. A silicon substrate,
An under clad arranged on this silicon substrate,
A first core made of silicon having a rectangular cross section disposed on the under cladding;
A taper portion made of silicon having a cross section whose cross-sectional area gradually decreases toward the tip, which is arranged integrally with the end of the first core on the under clad,
A second core disposed so as to cover the tapered portion;
An over clad disposed so as to cover the tapered portion and the second core,
The undercladding and the first core constitute a first optical waveguide,
The undercladding, the tapered portion, the second core, and the overcladding form a mode field size converter,
The under cladding, the second core, and the over cladding constitute a second optical waveguide,
An optical module, wherein a relative refractive index difference between the second core and the under cladding is larger than a relative refractive index difference between the second core and the over cladding.

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