JP2007079225A - Wavelength conversion element connection method and connection member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce connection loss in connection between a waveguide of a wavelength conversion element and an optical fiber having a different mode diameter.
A connecting member (15) for connecting a waveguide (23) formed in a quasi phase matching type wavelength conversion element (19) having a periodically poled structure and an optical fiber (14), which is optically coupled to the waveguide (23). A mode size conversion waveguide 22 having an end face that matches the mode diameter of the waveguide 23 and an end face that is optically coupled to the optical fiber 14 and matches the mode diameter of the optical fiber 14 is provided.
[Selection] Figure 4

Description

  The present invention relates to a wavelength conversion element connection method and connection member, and more particularly, to a wavelength conversion element connection method and connection member for optically coupling a waveguide formed in the wavelength conversion element and an optical fiber. About.

  Conventionally, as a wavelength conversion element that converts the wavelength of light, an element that uses a semiconductor optical amplifier, an element that uses four-wave mixing, second harmonic generation, sum frequency generation, and difference frequency generation that are second-order nonlinear optical effects A utilized wavelength conversion element or the like is known (for example, see Patent Document 1). Among them, a quasi phase matching wavelength conversion element (hereinafter referred to as a QPM-LN element) using lithium niobate (hereinafter referred to as LN) is incident light having one or two wavelengths input to the QPM-LN element. By changing the combination of the above and the period of the periodic domain-inverted structure, converted light having an arbitrary wavelength can be extracted. A light source using a QPM-LN element is expected to put a laser light source into practical use in the visible light region or the mid-infrared region.

  In order to modularize a light source using a QPM-LN element, it is necessary that the pumping light output from one or two semiconductor lasers is incident on the waveguide of the QPM-LN element via an optical fiber. . Therefore, when there is one semiconductor laser, a semiconductor laser having a fiber pigtail for output is prepared, and the fiber pigtail is connected to the waveguide of the QPM-LN element. When there are two semiconductor lasers, two pumping lights are multiplexed by a wavelength multiplexer / demultiplexer including an input / output optical fiber and connected to the waveguide of the QPM-LN element. In each case, the connection between the optical fiber and the waveguide is optically coupled by an optical system using a lens.

  For example, in order to generate yellow-green visible light having a wavelength of 560 nm, the excitation light output from a semiconductor laser having a wavelength of 1.3 μm and a semiconductor laser having a wavelength of 0.98 μm is combined using a wavelength multiplexer. Wave and enter the waveguide of the QPM-LN element. Visible light is output by taking out the converted light obtained by sum frequency generation of the QPM-LN element.

JP 2003-140214 A

  However, the optical circuit of an optical system using a lens has a problem that the number of parts such as a lens part, a holder for YAG welding the lens part, and a metal ferrule is large, and the work process for modularization is complicated. It was. There is also a problem that the price of individual parts such as lens parts is high and it is difficult to reduce the price of the module.

Further, when the above-described light source with a wavelength of 560 nm is configured, the mode defined by 1 / e ² of the light intensity is large because the wavelength fluctuation between the excitation light with a wavelength of 1.3 μm and the excitation light with a wavelength of 0.98 μm is large. Large difference in diameter. Therefore, in the case of coupling to the waveguide of the QPM-LN element, a loss occurs due to the difference in mode diameter. Further, since the wavelength fluctuation is large, there is a problem that the optimum coupling position varies depending on the wavelength due to the aberration of the lens.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a connection method and a connection member having a small connection loss between a waveguide of a wavelength conversion element and an optical fiber having a different mode diameter. There is.

  In order to achieve the above object, according to the present invention, the invention described in claim 1 connects a waveguide formed in a quasi phase matching type wavelength conversion element having a periodically poled structure and an optical fiber. A method of connecting wavelength conversion elements, wherein the connection is made via a mode size conversion optical circuit that matches the mode diameter of the waveguide with the mode diameter of the optical fiber.

  The invention according to claim 2 is a connecting member for connecting the waveguide formed in the quasi phase matching type wavelength conversion element having a periodically poled structure and an optical fiber, and optically coupled to the waveguide. And a mode size conversion waveguide having an end face that matches the mode diameter of the waveguide and an end face that is optically coupled to the optical fiber and matches the mode diameter of the optical fiber. To do.

  According to a third aspect of the present invention, the mode size conversion waveguide according to the second aspect is a tapered waveguide formed on a quartz substrate.

  The invention described in claim 4 is characterized in that the mode size conversion waveguide according to claim 2 is a tapered waveguide formed on a silicon substrate.

  According to a fifth aspect of the present invention, the mode size conversion waveguide according to the second aspect is a high relative refractive index difference optical fiber, and is connected to the optical fiber via a tapered fusion splicing portion. It is characterized by.

  According to a sixth aspect of the present invention, in the connecting member according to any of the second to fifth aspects, the waveguide formed in the quasi phase matching wavelength conversion element is a ridge type waveguide, It is a refractive index difference waveguide.

  The invention described in claim 7 is characterized in that the optical fiber according to any one of claims 2 to 5 is a polarization maintaining fiber.

  As described above, according to the present invention, the connection is made via the mode size conversion optical circuit that matches the mode diameter of the waveguide formed in the quasi phase matching wavelength conversion element with the mode diameter of the optical fiber. Therefore, connection loss can be reduced.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a method for connecting wavelength conversion elements according to Example 1 of the present invention. The waveguide of the wavelength conversion element 16 is connected to the optical fiber 14 fixed to the optical fiber fixing member 11 via the PLC 15 which is an optical circuit for mode size conversion.

  FIG. 2 shows a cross section of the optical fiber fixing member 11. The optical fiber fixing member 11 includes a substrate 12 made of quartz or Pyrex (registered trademark) in which a V groove 21 having a depth of 175 μm is formed, and a sheath 13. An optical fiber 14 having an outer diameter of 125 μm is held in the V-groove 21 and fixed with an adhesive, and the sheath 13 is fixed on the substrate 12 with an adhesive. The sheath 13 is a member for increasing the bonding area between the optical fiber fixing member 11 and the PLC 15. A circular optical fiber 14 is fixed so as to be supported at three points in a triangle formed by the slopes on both sides of the V-groove 21 and the bottom surface of the sheath 13.

  The optical fiber 14 is desirably a polarization maintaining fiber in order to make the plane of polarization incident on the waveguide of the QPM-LN element constant. Furthermore, it is desirable to use a polarization maintaining fiber (hereinafter referred to as 980PANDA fiber) that satisfies the single mode condition at a wavelength of 0.98 μm. The mode diameter of 1.3 μm light guided through the 980PANDA fiber is about 10 μm, whereas the mode diameter of 0.98 μm light is about 8 μm.

  FIG. 3 shows a cross section of the wavelength conversion element 16. In the wavelength conversion element 16, a QPM-LN element 19 is fixed on the base substrate 17 with an adhesive, and a sheath 18 is fixed on the upper part thereof with the adhesive. In the QPM-LN element 19, a waveguide 23 having a ridge structure is formed. The sheath 18 is a member for increasing the bonding area between the wavelength conversion element 16 and the PLC 15.

Member used in the base substrate 17 or Hire 18 members close thermal expansion coefficient to the value 17 × ¹⁰ -6 of LN (1 / K) is desirable in the sense to reduce the adhesion stress. When the thermal expansion coefficients are extremely different, for example, when quartz glass (thermal expansion coefficient <10 × 10 ⁻⁷ ) is used, thermal distortion occurs in the waveguide due to temperature rise or cooling after bonding, and in the worst case May damage the waveguide. Although it is most preferable to use an LN substrate, it is preferable to use a lithium tantalate (hereinafter referred to as LT) substrate (thermal expansion coefficient 15 × 10 ⁻⁶ ), a multicomponent glass (for example, Sumita Optical PFK85, PFK75 glass), or the like. it can.

  The thickness of the base substrate 17 or the sheath 18 is preferably 1 mm or more and 2 mm or less. As a suitable example, it is preferable to use a 1.5 mm LT substrate. If it is thinner than 1 mm, the strength is not sufficient, and there is a risk of breakage when fixed to a polishing jig that performs end face polishing. Also, a substrate thicker than 2 mm is expensive and expensive, and heat conduction to the waveguide is hindered, making it impossible to precisely control the temperature of the waveguide. Since the thickness of the QPM-LN element 19 is about 500 μm, it is most preferable that the base substrate 17 is 1.5 mm thick, the collar 18 is 1 mm thick, and the total thickness of the members left for end face bonding is 3 mm.

  FIG. 4 shows details of the connecting portion according to the first embodiment. The end face of the wavelength conversion element 16 is cut and polished at an angle of 12 degrees. The opposite end faces of the PLC 15 are polished at an angle of 18 degrees. This is because the refractive index of the QPM-LN element 19 is 2.1, whereas the refractive index of quartz glass is 1.48, and the direction of the waveguide is in the light propagation direction that is satisfied by Snell's law. This is because of matching. Further, the return light reflected from the end face can be reduced by cutting the end face of the waveguide obliquely.

  The PLC 15 is a quartz-based planar lightwave circuit manufactured on a quartz substrate or a silicon substrate. The core 22 produced in the PLC 15 has a buried structure, and the size of the core 22 is indicated by a dotted line in the drawing. The size of the core 22 inside the PLC 15 is such that the side coupled to the optical fiber 14 is thin and the side coupled to the waveguide 23 of the QPM-LN element 19 is thick.

  The end face of the optical fiber fixing member 11 is cut and polished at an angle of 8 degrees, and the end face of the opposite PLC 15 is also cut and polished at an angle of 8 degrees. In the case of being cut vertically, if the refractive index of quartz glass is 1.5, 4% (14 dB) of reflected return light is generated from the end face of the optical fiber 14 by Fresnel reflection. In order to obtain a return loss of 40 dB or more, it is necessary to cut the end face obliquely at an angle larger than 8 degrees.

  FIG. 5 shows the calculation result of the light intensity distribution at the connection portion according to the first example. Here, the relative refractive index difference (Δn) between the core and the cladding of the PLC 15 was set to 0.75%, and the relative refractive index difference (Δn) of the optical fiber 14 was set to 0.3%. FIG. 5A shows the intensity distribution of light on the PLC 15 side at the connection portion between the PLC 15 and the optical fiber fixing member 11. The calculation result of the intensity distribution of light having a wavelength of 1.3 μm when the width 2a of the core 15 of the PLC 15 is 1.2 μm and the height 2d is 6 μm is indicated by contour lines. The intensity distribution of light on the opposite optical fiber 14 side is shown in FIG. It can be understood that by reducing the core width of the PLC 15 to 1.2 μm, the intensity of light having a wavelength of 1.3 μm increases from the core 22 and substantially matches the mode diameter of the optical fiber 14. . As a result, the coupling loss is estimated to be -0.05 dB.

  FIG. 5C shows the intensity distribution of the light on the PLC 15 side at the connection portion between the PLC 15 and the wavelength conversion element 16. The calculation results of the intensity distribution of light having a wavelength of 1.3 μm when the core width 2a = 6 μm and the height 2d = 6 μm are shown by contour lines. On the other hand, the waveguide 23 of the QPM-LN element 19 has a ridge structure as shown in FIG. 3, and the three directions around the core are surrounded by an adhesive having a refractive index of 1.5. The waveguide has an ultrahigh specific refractive index difference Δn = 28%. The mode diameter of light having a wavelength of 1.3 μm in the waveguide 23 is almost the same as the waveguide size. In the present embodiment, the connection loss can be minimized when the size of the waveguide 23 of the QPM-LN element 19 is about 7 × 7 μm.

  In this way, by performing mode size conversion using a tapered waveguide, the connection loss is greatly reduced as compared with the case where the waveguide 23 of the QPM-LN element 19 and the optical fiber 14 are directly connected. be able to. Therefore, in the QPM-LN element 19, the amount of excitation light that contributes to wavelength conversion increases, and highly efficient wavelength conversion can be achieved.

FIG. 6 illustrates a method for manufacturing the QPM-LN element according to the second embodiment. In Example 2, a ridge waveguide type QPM-LN element 19 is manufactured using Zn-doped LiNbO ₃ . First, a Z-cut Zn-doped LiNbO ₃ substrate 41 and a Z-cut Mg-doped LiNbO ₃ substrate 42 on which a periodically poled structure is prepared in advance are prepared. The substrates 41 and 42 are both 3-inch wafers whose surfaces are optically polished and have a thickness of 500 μm.

  After making the surfaces of the substrates 41 and 42 hydrophilic by normal acid cleaning or alkali cleaning, the two substrates are superposed in a clean atmosphere. The superposed substrates are put in an electric furnace and bonded by heat treatment at 500 ° C. for 3 hours (first step). The bonded substrates are void-free, and cracks do not occur even when returned to room temperature. Next, the thickness of the substrate 41 is reduced to 8 μm using a grinding device such as a grinder and a polishing device. After polishing the substrate 41, polishing is performed to obtain a mirror-polished surface (second step).

  Next, the polished thin film substrate is set on a dicing saw, and a ridge waveguide having a core width of 7 μm is manufactured by precision processing using a diamond blade having a particle diameter of 4 microns or less (third step). The grooves 43a and 43b formed by dicing have a depth of about 20 μm. The depth of the grooves 43a and 43b is desirably deeper than the core film thickness, that is, 7 μm. When the film thickness is shallower than the core thickness, the waveguide's equivalent refractive index changes due to slight fluctuations in the groove depth in the direction of travel of the waveguide. Because. A preferable depth of the grooves 43a and 43b is about twice the thickness of the core to 40 μm or less. Processing deeper than 40 μm is because the mechanical strength of the waveguide itself deteriorates. A QPM-LN element 19 having a length of 20 mm is obtained by cutting the manufactured substrate into a strip shape and optically polishing the end face of the waveguide.

Note that the Z-cut Zn-doped LiNbO ₃ substrate 41 in advance periodically poled structure is formed, may be used and LT substrate. Further, the non-doped LiNbO ₃ substrate and the LT substrate may be used, or the QPM-LN element 19 can be manufactured by combining the Z-cut Mg-doped LiNbO ₃ substrate and the LT substrate.

  The thickness of the substrate is not limited to 500 μm, and a substrate having a thickness of 200 μm to 1 mm can be used. When the substrate thickness is 200 μm or less, voids may occur at the bonding interface during bonding due to warpage of the substrate itself. In addition, when a substrate thicker than 1 mm is used, the material cost of the substrate itself becomes high, which increases the manufacturing cost.

  Also in the second embodiment, the waveguide 23 of the QPM-LN element 19 and the optical fiber 14 are connected by the PLC 15 having a relative refractive index difference Δn = 0.75% between the core and the clad as in the first embodiment. The PLC 15 has a core width on the side connected to the optical fiber 14 of 1.2 μm, a core width on the side connected to the waveguide 23 of 6 μm, and a height of 6 μm, and the core width changes in a tapered shape.

  After aligning so that the connection loss between the optical fiber 14 and the core 22 is reduced, the PLC 15 and the optical fiber fixing member 11 are bonded and fixed with a UV adhesive. At this time, the connection loss of light having a wavelength of 1.3 μm is 0.05 dB. Next, after aligning the core 22 and the waveguide 23 so as to reduce the connection loss, the wavelength conversion element 16 is bonded and fixed. In the alignment at this time, light having a wavelength of 1.3 μm and light having a wavelength of 0.98 μm are combined by a fiber coupler and are incident on the optical fiber 14. While monitoring the yellow-green light of 560 nm generated by the sum frequency generation of the QPM-LN element 19, the wavelength conversion element 16 and the PLC 15 are bonded and fixed at the alignment position where the output becomes maximum.

  When light having a wavelength of 1.3 μm and 30 mW and light having a wavelength of 0.98 μm and 70 mW are input, output light having a wavelength of 560 nm and 25 mW can be obtained, and a high conversion efficiency of 1200% / W or more can be obtained.

  A PLC 15 having a relative refractive index difference Δn = 1.5% can also be used. When the core width on the side connected to the optical fiber 14 which is a 980PANDA fiber is 1.0 μm and the height is 4.5 μm, the coupling loss can be minimized. By setting the core width of the QPM-LN element 19 on the side connected to the waveguide 23 to 4.5 μm, the height to 4.5 μm, and the core size of the waveguide 23 to 5 × 5 μm, 2000% / W Wavelength conversion efficiency can be obtained.

  FIG. 7 shows a method for connecting wavelength conversion elements according to Example 3 of the present invention. In the third embodiment, a high relative refractive index difference optical fiber is used instead of the PLC as an optical circuit for mode size conversion. The waveguide 23 of the QPM-LN element 19 is connected to a high relative refractive index difference optical fiber 54 fixed to the optical fiber fixing member 51. The optical fiber fixing member 51 includes a substrate 52 made of quartz or Pyrex (registered trademark) in which a V-groove 55 is formed, and an unillustrated (not shown). The end face of the wavelength conversion element 16 is cut and polished at an angle of 12 degrees. The end surfaces of the opposing optical fiber fixing members 51 are polished at an angle of 18 degrees.

  The high relative refractive index difference optical fiber 54 is fused and connected to the 980 PANDA fiber 62. The fusion splicing portion 63 is processed into a taper shape by TEC processing so that the core diameter gradually changes, and is fixed by the reinforcing member 61. FIG. 7 conceptually shows how the core diameter of the optical fiber changes at the fusion splicing portion 63, and the outer diameter of the optical fiber is not shown.

  The high relative refractive index difference optical fiber 54 has Δn = 1.9%, and the mode diameter of light having a wavelength of 1.3 μm is 5.0 μm. Therefore, when the core size of the waveguide 23 of the QPM-LN element 19 is 5 × 5 μm, the connection loss can be minimized.

  When light having a wavelength of 1.3 μm and 30 mW and light having a wavelength of 0.98 μm and 70 mW are input, output light having a wavelength of 560 nm and 25 mW can be obtained, and a high conversion efficiency of 1200% / W or more can be obtained. When the high relative refractive index difference optical fiber 54 with Δn = 2.2% is used, the core size of the waveguide 23 is 4 × 4 μm, and a conversion efficiency of 2000% / W can be obtained. Furthermore, when the high relative refractive index difference optical fiber 54 with Δn = 3.7% is used, the core size of the waveguide 23 is 3 × 3 μm, and a conversion efficiency of 2500% / W can be obtained.

It is a block diagram which shows the connection method of the wavelength conversion element concerning Example 1 of this invention. It is sectional drawing which shows the structure of an optical fiber fixing member. It is sectional drawing which shows the structure of a wavelength conversion element. It is a figure which shows the detail of the connection part concerning Example 1. FIG. It is the figure which showed the calculation result of the light intensity distribution of the connection part concerning Example 1 by the contour line. 6 is a diagram showing a method for manufacturing a QPM-LN element according to Example 2. FIG. It is a block diagram which shows the connection method of the wavelength conversion element concerning Example 3 of this invention.

Explanation of symbols

11, 51 Optical fiber fixing member 12, 52 Substrate 13, 18 Yayoi 14 Optical fiber 15 PLC
16 Wavelength conversion element 17 Base substrate 19 QPM-LN element 21, 55 V groove 22 Core 23 Waveguide 54 High relative refractive index difference optical fiber 61 Reinforcement member 62 980PANDA fiber 63 Fusion splicing part

Claims (7)

A wavelength conversion element connection method for connecting a waveguide formed in a quasi-phase matching type wavelength conversion element having a periodically poled structure and an optical fiber,
A method of connecting wavelength conversion elements, comprising: connecting via a mode size conversion optical circuit that matches a mode diameter of the waveguide and a mode diameter of the optical fiber. A connecting member that connects an optical fiber and a waveguide formed in a quasi phase matching wavelength conversion element having a periodically poled structure,
A mode size conversion guide having an end face optically coupled to the waveguide and matching the mode diameter of the waveguide, and an end face optically coupled to the optical fiber and matching the mode diameter of the optical fiber. A connection member comprising a waveguide.   The connection member according to claim 2, wherein the mode size conversion waveguide is a tapered waveguide formed on a quartz substrate.   The connection member according to claim 2, wherein the mode size conversion waveguide is a tapered waveguide formed on a silicon substrate.   The connection member according to claim 2, wherein the mode size conversion waveguide is a high relative refractive index difference optical fiber, and is connected to the optical fiber via a tapered fusion splicing portion. .   6. The connection according to claim 2, wherein the waveguide formed in the quasi phase matching type wavelength conversion element is a ridge type waveguide and is a high relative refractive index difference waveguide. Element. The connection member according to claim 2, wherein the optical fiber is a polarization maintaining fiber.

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